Collections: Fortification, Part IV: French Guns and Italian Lines

This is the fourth part of a five part (I, II, III) series covering some of the basics of fortification, from city walls to field fortifications, from the ancient world to the modern period. Last week, we set out an overview of fortifications in medieval Europe, with particular focus on the strategic role of castles and point defenses.

This week, we’re going to look at the impact artillery has on these systems. So far the fortifications we have been looking at have mostly been designed to resist escalade – enemies coming over the walls, by whatever means. Indeed, on the ‘list of threats’ medieval and ancient fortifications were clearly more concerned about escalade, treachery and starvation; breaching was clearly a secondary concern, with some design implications we’ll talk about here shortly. That was a sensible set of priorities because, as we’ll see, until the development of mature gunpowder artillery in the mid-15th century, artillery (by which we mean catapults), while they existed and could be very useful in a siege, were not generally up to the task of actually breaching a castle’s curtain wall.

Gunpowder decisively changes that and as a result transforms fortifications, though as we’ll see, it doesn’t do so uniformly either in design or geographically.

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Bibliography Note! As we’ve discussed in the past, trace italienne fortresses (the sort we’re going to talk about here) sit at the center of the debate over the ‘military revolution’ – the source of the sudden increase in military capacity tied to gunpowder in Europe from about 1450 to about 1750. Consequently, there is a lot of bibliography on them. This post owes its data to several key works, namely, T. Andrade, The Gunpowder Age: China, Military Innovation and the Rise of the West in World History (2016), G. Parker, The Military Revolution: Military Innovation and the Rise of the West 1500-1800 (1988; 2nd ed. 1996), and G. Parker, The Army of Flanders and the Spanish Road, 1567-1659 (1972; 2nd ed. 2004). There’s a lot more on this, but that’s a good place to start. Since we’re also discussing catapults, I should note that the standard work on Greek and Roman catapults remains E.W. Marsden, Greek and Roman Artillery: Historical Developments (1969), while the most recent monograph discussion of medieval European catapults that I know of is M. Fulton, Artillery in the Era of the Crusades: Siege Warfare and the Development of Trebuchet Technology (2018); alas, neither book is particularly affordable, so see if you can’t get your library to swing you a copy (Marsden, in particular, is difficult to find; note that it comes in two volumes – you want Historical Developments, rather than Technical Treatises). There is a decent discussion of the use of catapults in sieges in C. Rogers, Soldiers’ Lives Through History: The Middle Ages (2007), 121-4. Towards the end, we briefly touch on the question of firepower in the American Civil War. The key reading here is E.J. Hess, The Rifle Musket in Civil War Combat: Reality and Myth (2008) and E.J. Hess, Field Armies and Fortifications in the Civil War: The Eastern Campaign 1861-1864 (2005), but note also B. Gibbons, The Destroying Angel: The Rifle-Musket as the First Modern Infantry Weapon (2019). Finally, for a fairly approachable overview of the changing nature of warfare in this period, see Lee, Waging War: Conflict, Culture and Innovation in World History (2016), ch. 7-11.

Catapults (…Aren’t in Chess, So They Go Here)

Cannon weren’t the first form of artillery used to batter fortifications, so before we get to gunpowder it is worth backing up and discussing catapults and the sort of ‘artillery threat’ that catapults create. And here once again we need to clarify some terms: catapults are generally defined by the mechanism they use to store and then release energy, because that is fundamentally what a catapult is: a device for storing up some energy and then releasing it very suddenly to propel a large object.

The very oldest catapults, first invented by the Greeks were tension catapults (the gastrophetes and oxybeles), which functioned like large bows, with a bow-staff being bent backwards to store and then release the launching energy. This sort of design, common in pop-cultural depictions of catapults, is actually quite limited as with the materials available, there is a real limit to how much energy can be stored via tension. Fortunately for the Greeks, by the early fourth century, they had developed a better method.

Instead, the Greeks, Macedonians and Romans began using torsion catapults (where the energy is stored in wound-up sinews like a spring). While the devices used in field battles (and for city defense) were often smaller, arrow-launching devices, siege catapults could be very large; the standard engine for the purpose could fling a 1 talent stone (26.2kg) about 400m (though effectiveness was far higher if you could get closer to the wall, which as we’ll see will be a trend for most of this post); much larger engines did exist as well. That said, Roman catapults were mostly not for collapsing walls but for destroying towers and suppressing defenders in order to aid in escalade (usually by mole, rather than ladders or towers, though the Romans used those too).

Via Wikipedia, a selection of small reconstructed Greek and Roman catapults from Saalburg. Roman siege catapults came much larger than this

And here once again the distinction between the ‘big army siege package’ and the ‘small army siege package’ matters quite a bit. Roman torsion artillery was complex, expensive and required lots of technical skill, and so sees far diminished use in the early Middle Ages where that technical skill is hard to come by. Vespasian, we are told, brought 160 torsion catapults to besiege Jotapata in 67 (Josephus BJ 3.166) while Titus brings a stunning 340 to besiege Jerusalem in 70 (Josephus BJ 5.356). By contrast, the construction of a single catapult is often a major event in a medieval siege (see Rogers, op. cit. 121-3 for some examples) and while later medieval catapults were often more powerful than the earlier Roman torsion devices, they were not that much more powerful.

Consequently, Hellenistic and Roman fortifications (especially city walls, like the Theodosian Walls we discussed last time) were designed with massed catapults in mind. As noted, the multiple walls ensured that the main curtain wall, the inner wall, was extremely difficult to target with catapults or indeed any kind of artillery: even if you knocked down the low wall and the outer wall, their rubble would mostly block shots at the base of the inner wall. Meanwhile, the inner wall was built to be practically immune to catapult fire anyway: up to 6m thick without any internal passages (the outer wall was much thinner, only 2m). That was more than enough to render the walls effectively immune to anything catapults can do; the walls in many places still stood up to Ottoman cannon in 1453. Finally, ancient city defenses were built assuming they’d often have their own stone and arrow throwing torsion artillery set up on the towers to return ‘counter-battery’ fire. Not every city had the ‘complete package’ that Constantinople, as the imperial capital head, of course, but some mix of thick walls, low out-walls and catapults designed for counter-battery fire were fairly standard defensive arrangements for Roman cities that could afford them and felt sufficiently threatened to invest the resources.

As we move into the Middle Ages, two paradoxical things happen. On the one hand, the ability for societies in Europe to deploy large numbers of finicky, high-tech torsion artillery decreases dramatically (and the machines that we do see tend to be the simpler, less accurate single-armed variety, what the Romans called the onager or ‘wild ass’ because it kicked like one when it fired). On the other hand, by the sixth century, we start to see a clever new design of catapult, the traction trebuchet.

Originating in China in the 4th century BC, the traction catapult used muscle power directly to swing a long pole around a central frame. In terms of engineering complexity, it was a simpler device, and could be scaled up quite large so long as one could add more pullers (around 100 seems to have been normal for a large engine), but the range and power it offered as a result of the mechanical advantage offered by the long throwing arm were considerable. Given the number of pullers required, it is little surprise these were generally only used in small numbers in medieval Europe (again, often in reports it is merely a single device, described as a mangonel or a fenevol), but on the other hand, as I understand the physics, the range and striking power had the potential to be superior to a torsion catapult. Nevertheless, if we look at the kinds of fortifications emerging during this period, it certainly seems like in Europe, the concern that artillery might produce a breach in the wall (as opposed to merely degrading towers and the wall-walk) was fairly low.

Via Wikipedia, a 12th century manuscript illustration showing the use of a traction trebuchet (note the ropes handing down from the short end of the rod for pulling on.

Just to throw down a note here because we’ll come back to it, it is striking that while the small numbers of traction trebuchets in Europe seem to have represented a decline in the ‘catapult threat’ to walls (recall last week’s contrast between castle walls and the much older Theodosian Walls), that was not the case in China, where walls continued to be made very thick – a design quirk that will matter quite a lot in a moment. I am not an expert on ancient and medieval Chinese siege tactics, alas, but my brief encounters with accounts of them often seem to describe traction catapults used en masse, in dozens or even hundreds, much more the way that the Romans used massed siege artillery. Likewise, Michael Fulton (Artillery in the Era of the Crusades (2018)) notes nearly a hundred Mamluk trebuchets (a mix of counter-weight and traction) at the Siege of Acre (1291); my sense is that such large siege trains were very rare within Europe. Presumably the ability to deploy so many engines was a consequence of greater state capacity in China and the Near East during this period as compared to fragmented, decentralized medieval Europe.

The late 12th century sees a major variation on the trebuchet design: the use of a counter-weight, instead of traction to provide the force; this innovation seems to have emerged in the West broadly defined, though it isn’t clear if that means in Europe or the Middle East (in any event both Christian and Muslim armies start using them at almost exactly the same time). This allows for much more energy to put into the shot, as the counter-weight can be very heavy and only slowly winched into place, allowing the work crew to spend more time ‘storing’ energy in the counter-weight than they could with the quick pull of a traction trebuchet. Larger counter-weight trebuchets could also make use of animals to provide the power, or large wheels to make it easier to raise the counter-weight. The upper-limits on the size of projectiles were very high: Warwolf is thought to be the largest such trebuchet known, and threw a nearly 300lbs shot. That said, while counter-weight trebuchets hit harder (but fired slower), in function they do not seem to have been meaningfully different from traction trebuchets; they were used the same way in sieges.

Via Wikipedia, a diagram of a large counterweight trebuchet.

What’s really striking is not the vast impact of catapults, but the muted impact of catapults. The counter-weight trebuchet was clearly good: the innovation makes its way all the way back to China, carried by the Mongols who presumably picked it up in the Middle East (ironically moving the opposite direction but at the same time as gunpowder, suggesting that at this point in the 13th century the two technologies were not considered mutually exclusive). Castle design does respond to catapults, but only in relatively modest ways: walls get somewhat thicker, but as Fulton (op cit.) notes, only by about half a meter or so (leaving even the newly thickened medieval castle walls somewhat thinner than the best old Roman defenses). In at least some areas, towers and keeps become more frequently rounded in shape, to resist catapult fire.

Certainly it was possible for catapults to open breaches in weaker walls to enable assault. The aforementioned Warwolf opened large breaches in the stone walls of Stirling Castle in 1304. But I note both Rogers (op. cit.) and Fulton (op. cit.) seem to confirm that while true breaches from trebuchets could happen, it was far more common that walls resisted trebuchet strikes and that the real work of the machines was degrading the wall defenses by striking off battlements and smashing towers, in order to enable escalade. Which is little surprise: that’s precisely what the Romans used catapults for too. While there is still some argument about the degree to which the counter-weight trebuchet was a revolutionary military technology, on the balance, the siege playbook changed only modestly to accommodate it, and castle design likewise shifted only in degrees.

And then Charles VIII of France (r. 1483-1498) decided to take a holiday on the Bay of Naples.

Chuck’s Italian Vacation

Of course that last line is a click-bait oversimplification. In fact what we should say here is that the basic formula for what would become gunpowder (saltpeter, charcoal and sulfur in a roughly 75%, 15%, 10% mixture) was clearly in use in China by 1040 when we have our first attested formula, though saltpeter had been being refined and used as an incendiary since at least 808. The first guns appear to be extrapolations from Chinese incendiary ‘fire lances’ (just add rocks!) and the first Chinese cannon appear in 1128. Guns arrive in Europe around 1300; our first representation of a cannon is from 1326, while we hear about them used in sieges beginning in the 1330s; the Mongol conquest and the sudden unification of the Eurasian Steppe probably provided the route for gunpowder and guns to move from China to Europe. Notably, guns seem to have arrived as a complete technology: chemistry, ignition system, tube and projectile.

There was still a long ‘shaking out’ period for the new technology: figuring out how to get enough saltpeter for gunpowder (now that’s a story we’ll come back to some day), how to build a large enough and strong enough metal barrel, and how to actually use the weapons (in sieges? against infantry? big guns? little guns?) and so on. By 1453, the Ottomans have a capable siege-train of gunpowder artillery. Mehmed II (r. 1444-1481) pummeled the walls of Constantinople with some 5,000 shots using some 55,000 pounds of gunpowder; at last Theodosius’ engineers had met their match.

Via Wikipedia, Mons Meg, a large hoop-and-stave constructed cannon built in Burgundy in 1449 and sent to Scotland in 1454.

And then, in 1494, Charles VIII invaded Italy – in a dispute over the throne of Naples – with the first proper mobile siege train in Christendom (not in Europe, mind you, because Mehmed had beat our boy Chuck here by a solid four decades). A lot of changes had been happening to make these guns more effective: longer barrels allowed for more power and accuracy, wheeled carriages made them more mobile, trunnions made elevation control easier and some limited degree of caliber standardization reduced windage and simplified supply (though standardization at this point remains quite limited).

The various Italian states, exactly none of whom were excited to see Charles attempting to claim the Kingdom of Naples, could have figured that the many castles and fortified cities of northern Italy were likely to slow Charles down, giving them plenty of time to finish up their own holidays before this obnoxious French tourist showed up. On the 19th of October, 1494, Charles showed up to besiege the fortress at Mordano – fortress which might well have been expected to hold him up for weeks or even months; on October 20th, 1494, Charles sacked Mordano and massacred the inhabitants, after having blasted a breach with his guns. Florence promptly surrendered and Charles marched to Naples, taking it in 1495 (it surrendered too). Francesco Guicciardini phrased it thusly (trans. via Lee, Waging War, 228),

They [Charles’ artillery] were planted against the Walls of a Town with such speed, the Space between the Shots was so little, and the Balls flew so quick, and were impelled with such Force, that as much Execution was done in a few Hours, as formerly, in Italy, in the like Number of Days.

The impacts of the sudden apparent obsolescence of European castles were considerable. The period from 1450 to 1550 sees a remarkable degree of state-consolidation in Europe (broadly construed)1 castles, and the power they gave the local nobility to resist the crown, had been one of the drivers of European fragmentation, though we should be careful not to overstate the gunpowder impact here: there are a other reasons for a burst of state consolidation at this juncture. Of course that creates a run-away effect of its own, as states that consolidate have the resources to employ larger and more effective siege trains.

Now this strong reaction doesn’t happen everywhere or really anywhere outside of Europe. part of that has to do with the way that castles were built. Because castles were designed to resist escalade, the walls needed to be built as high as possible, since that was the best way to resist attacks by ladders or towers. But of course, given a fixed amount of building resources, building high also means building thin (and European masonry techniques enabled tall-and-thin construction with walls essentially being constructed with a thick layer of fill material sandwiched between courses of stone). But ‘tall and thin,’ while good against ladders, was a huge liability against cannon.

By contrast, city walls in China were often constructed using a rammed earth core. In essence, earth was piled up in courses and packed very tightly, and then sheathed in stone. This was a labor-intensive building style (but large cities and lots of state capacity meant that labor was available), and it meant the walls had to be made thick in order to be made tall since even rammed earth can only be piled up at an angle substantially less than vertical. But against cannon, the result was walls which were already massively thick, impossible to topple over and the earth-fill, unlike European stone-fill, could absorb some of the energy of the impact without cracking or shattering. Even if the stone shell was broken, the earth wouldn’t tumble out (because it was rammed), but would instead self-seal small gaps. And no attacker could hope that a few lucky hits to the base of a wall built like this would cause it to topple over, given how wide it is at the base. Consequently, European castle walls were vulnerable to cannon in a way that contemporary walls in many other places, such as China, were not. Again, path dependence in fortification matters, because of that antagonistic co-evolution.

Via Wikipedia, an image of a defensive wall in Xi’an, which has been cut through so you can see the interior. You could shoot cannon at this all day and not get very far: breaching this would take a long time.

In the event, in Italy, Charles’ Italian Vacation started to go badly almost immediately after Naples was taken. A united front against him, the League of Venice, formed in 1495 and fought Charles to a bloody draw at Fornovo in July, 1495. In the long-term, French involvement would draw in the Habsburgs, whose involvement would prevent the French from making permanent gains in a series of wars in Italy lasting well into the 1550s.

But more relevant for our topic was the tremendous shock of that first campaign and the sudden failure of defenses which had long been considered strong. The reader can, I’d argue, detect the continued light tremors of that shock as late as Machiavelli’s The Prince (1532, but perhaps written in some form by 1513). Meanwhile, Italian fortress designers were already at work retrofitting old castles and fortifications (and building new ones) to more effectively resist artillery. Their secret weapon? Geometry.

Math Fort

Now I should note that the initial response in Italy to the shocking appearance of effective siege artillery was not to immediately devise an almost entirely new system of fortifications from first principles, but rather – as you might imagine – to hastily retrofit old fortresses. But for the sake of keeping complication in what is already bound to be a long post reasonably limited, we’re going to focus on the eventual new system of fortresses which emerge, with the first mature examples appearing around the first decades of the 1500s in Italy. This system of European gunpowder fort that spreads throughout much of Europe and into the by-this-point expanding European imperial holdings abroad (albeit more unevenly there) goes by a few names: ‘bastion’ fort (functional, for reasons we’ll get to in a moment), ‘star fort’ (marvelously descriptive), and the trace italienne or ‘the Italian line.’ since that was where it was from.

Since the goal remains preventing an enemy from entering a place, be that a city or a fortress, the first step has to be to develop a wall that can’t simply be demolished by artillery in a good afternoon or two. The solution that is come upon ends up looking a lot like those Chinese rammed earth walls: earthworks are very good at absorbing the impact of cannon balls (which, remember, are at this point just that: stone and metal balls; they do not explode yet): small air pockets absorb some of the energy of impact and dirt doesn’t shatter, it just displaces (and not very far: again, no high explosive shells, so nothing to blow up the earthwork). Facing an earthwork mound with stonework lets the earth absorb the impacts while giving your wall a good, climb-resistant face.

So you have your form: a stonework or brick-faced wall that is backed up by essentially a thick earthen berm like the Roman agger. Now you want to make sure incoming cannon balls aren’t striking it dead on: you want to literally play the angles. Inclining the wall slightly makes its construction easier and the end result more stable (because earthworks tend not to stand straight up; note the rammed-earth core wall pictured above and how its sides angle) and gives you an non-perpendicular angle of impact from cannon when they’re firing at very short range (and thus at very low trajectory), which is when they are most dangerous since that’s when they’ll have the most energy in impact. Ideally, you’ll want more angles than this, but we’ll get to that in a moment.

Because we now have a problem: escalade. Remember escalade?

Crap, he’s back. Just don’t make eye contact, and he’ll go away.
I am going to keep making this joke until one of you laughs. I am not proud. Or tired.

Earthworks need to be wide at the base to support a meaningful amount of height, tall-and-thin isn’t an option. Which means that in building these cannon resistant walls, for a given amount of labor and resources and a given wall circuit, we’re going to end up with substantially lower walls. We can enhance their relative height with a ditch or several out in front (and we will), but that doesn’t change the fact that our walls are lower and also that they now incline backwards slightly, which makes them easier to scale or get ladders on. But obviously we can’t achieved much if we’ve rendered our walls save from bombardment only to have them taken by escalade. We need some way to stop people just climbing over the wall.

The solution here is firepower. Whereas a castle was designed under the assumption the enemy would reach the foot of the wall (and then have their escalade defeated), if our defenders can develop enough fire, both against approaching enemies and also against any enemy that reaches the wall, they can prohibit escalade. And good news: gunpowder has, by this point, delivered much more lethal anti-personnel weapons, in the form of lighter cannon but also in the form of muskets and arquebuses. At close range, those weapons were powerful enough to defeat any shield or armor a man could carry, meaning that enemies at close range trying to approach the wall, set up ladders and scale would be extremely vulnerable: in practice, if you could get enough muskets and small cannon firing at them, they wouldn’t even be able to make the attempt.

Via Wikipedia, a bastion (the ‘home plate’ shaped hollow pentagram) at the fortress of Komárno, Slovakia.

But the old projecting tower of the castle, you will recall, was designed to allow only a handful of defenders fire down any given section of wall; we still want that good enfilade fire effect, but we need a lot more space to get enough muskets up there to develop that fire. The solution: the bastion. A bastion was an often diamond or triangular-shaped projection from the wall of the fort, which provided a longer stretch of protected wall which could fire down the length of the curtain wall. It consists of two ‘flanks’ which meet the curtain wall and are perpendicular to it, allowing fire along the wall; the ‘faces’ (also two) then face outward, away from the fort to direct fire at distant besiegers. When places at the corners of forts, this setup tends to produce outward-spiked diamonds, while a bastion set along a flat face of curtain wall tends to resemble an irregular pentagon (‘home plate’) shape. The added benefit for these angles? From the enemy siege lines, they present an oblique profile to enemy artillery, making the bastions quite hard to batter down with cannon, since shots will tend to ricochet off of the slanted line.

Via Wikipedia, the Castillo de San Marcos in Florida, a fairly simple bastion fort with four bastions. The base of each diamond (closest to the curtain) allows for firing directly along the base of the curtain, while the longer tip provides positions for muskets or cannon (mostly the latter) to be directed outward against besiegers at distance (approaching infantry or enemy artillery).

In the simplest trace italienne forts, this is all you will need: four or five thick-and-low curtain walls to make the shape, plus a bastion at each corner (also thick-and-low, sometimes hollow, sometimes all at the height of the wall-walk), with a dry moat (read: big ditch) running the perimeter to slow down attackers, increase the effective height of the wall and shield the base of the curtain wall from artillery fire.

But why stay simple, there’s so much more we can do! First of all, our enemy, we assume, have cannon. Probably lots of cannon. And while our walls are now cannon resistant, they’re not cannon immune; pound on them long enough and there will be a breach. Of course collapsing a bastion is both hard (because it is angled) and doesn’t produce a breach, but the curtain walls both have to run perpendicular to the enemy’s firing position (because they have to enclose something) and if breached will allow access to the fort. We have to protect them! Of course one option is to protect them with fire, which is why our bastions have faces; note above how while the flanks of the bastions are designed for small arms, the faces are built with cannon in mind: this is for counter-battery fire against a besieger, to silence his cannon and protect the curtain wall. But our besieger wouldn’t be here if they didn’t think they could decisively out shoot our defensive guns.

Via Wikipedia, Fort McHenry, in Baltimore, Maryland. This is the first trace italienne fort I ever visited (not that I knew that was what it was called at the time. The triangle bit blocking the entrance to the gate is a ravelin. By the by, for my American readers: there are a lot of good bastion forts in the States and Canada, but they’re generally much smaller (as you’ll see in a moment) than their counterparts in Europe.

But we can protect the curtain further, and further complicate the attack with outworks, effectively little mini-bastions projecting off of the main wall which both provide advanced firing positions (which do not provide access to the fort and so which can be safely abandoned if necessary) and physically obstruct the curtain wall itself from enemy fire. The most basic of these was a ravelin (also called a ‘demi-lune’), which was essentially a ‘flying’ bastion – a triangular earthwork set out from the walls. Ravelins are almost always hollow (that is, the walls only face away from the fort), so that if attackers were to seize a ravelin, they’d have no cover from fire coming from the main bastions and the curtain wall.

And now, unlike the Modern Major-General, you know what is meant by a ravelin…but are you still, in matters vegetable, animal and mineral, the very model of a modern Major-General?

Via Wikipedia, Palmanova, Italy, famous for being an example of a pre-planned, ‘ideal’ Renaissance city. Renaissance thinking about cities frequently fell on the idea of the ‘star shaped’ city – one easily circled by a bastion fortress – as the ideal shape of a city, with a large central plaza, as here.
Also note how the main curtain wall and its bastions are supported by a series of ravelins and even further off, several redans beyond the moat and glacis.

But we can take this even further (can you tell I just love these damn forts?). A big part of our defense is developing fire from our bastions with our own cannon to force back enemy artillery. But our bastions are potentially vulnerable themselves; our ravelins cover their flanks, but the bastion faces could be battered down. We need some way to prevent the enemy from aiming effective fire at the base of our bastion. The solution? A crownwork. Essentially a super-ravelin, the crownwork contains a full bastion at its center (but lower than our main bastion, so we can fire over it), along with two half-bastions (called, wait for it, ‘demi-bastions’) to provide a ton of enfilade fire along the curtain wall, physically shielding our bastion from fire and giving us a forward fighting position we can use to protect our big guns up in the bastion. A smaller version of the crownwork, called a hornwork can also be used: this is just the two half-bastions with the full bastion removed, often used to shield ravelins (so you have a hornwork shielding a ravelin shielding the curtain wall shielding the fort). For good measure, we can connect these outworks to the main fort with removable little wooden bridges so we can easily move from the main fort out to the outworks, but if the enemy takes an outwork, we can quickly cut it off and – because the outworks are all made hollow – shoot down the attackers who cannot take cover within the hollow shape.

Via Wikipedia, an ideal form of a bastion fortress to show each kind of common work and outwork.

We can also do some work with the moat. By adding an earthwork directly in front of it, which arcs slightly uphill, called a glacis, we can both put the enemy at an angle where shots from our wall will run parallel to the ground, thus exposing the attackers further as they advance, and create a position for our own troops to come out of the fort and fire from further forward, by having them crouch in the moat behind the glacis. Indeed, having prepared, covered forward positions (which are designed to be entirely open to the fort) for firing from at defenders is extremely handy, so we could even put such firing positions – set up in these same, carefully mathematically calculated angle shapes, but much lower to the ground – out in front of the glacis; these get all sorts of names: a counterguard or couvreface if they’re a simple triangle-shape, a redan if they have something closer to a shallow bastion shape, and a flèche if they have a sharper, more pronounced face. Thus as an enemy advances, defending skirmishers can first fire from the redans and flèches, before falling back to fire from the glacis while the main garrison fires over their heads into the enemy from the bastions and outworks themselves.

Via Wikipedia, a diagram showing a glacis supporting a pair of bastions, one hollow, one not.

At the same time, a bastion fortress complex might connect multiple complete circuits. In some cases, an entire bastion fort might be placed within the first, merely elevated above it (the term for this is a ‘cavalier’) so that both could fire, one over the other. Alternately, when entire cities were enclosed in these fortification systems (and that was common along the fracture zones between the emerging European great powers), something as large as a city might require an extensive fortress system, with bastions and outworks running the whole perimeter of the city, sometimes with nearly complete bastion fortresses placed within the network as citadels.

Via Wikipedia, Fort Saint-Nicolas, which dominates the Old Port of Marseille. The fort forms part of a system with the low outwork you see here and also an older refitted castle, Fort Saint-Jean, on the other side of the harbor. I was able to see this one in person (its even bigger up close), but alas, while there are vague plans to open Fort Saint-Nicolas to the public, they had not proceeded very far when I was there. Marseillais, if this changes, tell me!

All of this geometry needed to be carefully laid out to ensure that all lines of approach were covered with as much fire as possible and that there were no blindspots along the wall. That in turn meant that the designers of these fortresses needed to be careful with their layout: the spacing, angles and lines all needed to be right, which required quite a lot of math and geometry to manage. Combined with the increasing importance of ballistics for calculating artillery trajectories, this led to an increasing emphasis on mathematics in the ‘science of warfare,’ to the point that some military theorists began to argue (particularly as one pushes into the Enlightenment with its emphasis on the power of reason, logic and empirical investigation to answer all questions) that military affairs could be reduced to pure calculation, a ‘hard science’ as it were, a point which Clausewitz (drink!) goes out of his way to dismiss (as does Ardant du Picq in Battle Studies, but at substantially greater length). But it isn’t hard to see how, in the heady centuries between 1500 and 1800 how the rapid way that science had revolutionized war and reduced activities once governed by tradition and habit to exercises in geometry, one might look forward and assume that trend would continue until the whole affair of war could be reduced to a set of theorems and postulates. It cannot be, of course – the problem is the human element (though the military training of those centuries worked hard to try to turn men into ‘mechanical soldiers’ who could be expected to perform their role with the same neat mathmatical precision of a trace italienne ravelin). Nevertheless this tension – between the science of war and its art – was not new (it dates back at least as far as Hellenistic military manuals) nor is it yet settled.

Via Wikipedia, an aerial view of the Bourtange Fortress in Groningen, Netherlands. Built in 1593, the fort has currently been restored for its 1750s configuration, seen here.

But coming back to our fancy forts, of course such fortresses required larger and larger garrisons to fire all of the muskets and cannon that their firepower oriented defense plans required. Fortunately for the fortress designers, state capacity in Europe was rising rapidly and so larger and larger armies were ready to hand. That causes all sorts of other knock on effects we’re not directly concerned with here (but see the bibliography at the top). For us, the more immediate problem is, well, now we’ve built one of these things…how on earth does one besiege it?

Siege Math

Of course part of the answer to this question was ‘you don’t.’ Areas that became densely set with trace italienne fortresses – particularly the towns of the Low Countries – became almost impossible to conquer. The Army of Flanders, then arguably the finest in Europe, tried for eighty years to subdue the Dutch and largely failed: the cost of endless sieges of trace italienne fortified towns made the task effectively hopeless. Once again, I don’t want to imply that this is the only factor (it isn’t, by any means), but after the long series of remarkably decisive wars from 1450 to 1550, the trace italienne contributed to the frustratingly inconclusive (but expensive and bloody) wars of the following century.

At the same time, this shift back towards defensive stalemate didn’t lead towards more fragmentation – as the castle had – because trace italienne fortresses were too expensive for any individual aristocrat to build merely to protect his house. Not merely because of the massive fortifications themselves, but the large garrisons they required, either of full-time soldiers or town militia, which were simply beyond the resources of what was left of the old medieval aristocracy. Indeed, warfare in this period (both offensive and defensive) largely proved to be beyond the resources of Europe’s newly centralizing states; the Spanish crown, for instance, went bankrupt, primarily from military expenses, in 1557, 1560, 1575, 1596, 1607, 1627, 1647 and 1653.

It is of part and parcel of the era that as much as building and defending a trace italienne fortress was an exercise in mathematics, so was attacking one. Because the entire point of the fortress is to project firepower – often at great distances (with cannon) – the attacker cannot simply form up outside or just set up their artillery in an open field and begin firing; they’d be cut up by counter-battery fire before they had gotten very far. So the attacker had to set up their own earthworks, which naturally being based on the same principles and weapons as the defender’s, would resemble them.

First, the attacking army would generally set up its own fortified camp, outside of cannon-shot. Because that camp needed to be resistant to enemy attack either from the garrison of a relieving army, it was likely to be built in the same style as a trace italienne fortification, albeit with earthworks and gabions (wicker baskets filled with earth) in place of the heavier stonework of the fort. The attacker then has to isolate their target from help, preventing the defenders from leaving, or supplies or reinforcements from arriving. The act of enclosing a besieged settlement in a wall is called circumvallation (recall our word vallum there; this is ‘walling around’). The act of constructing a line of outward facing defenses to defend that line from attack by a relieving army from behind is called contravallation. Armies in this period would do both.

Diagram of the Siege of Breisach, 1638, showing both circumvallation and contravallation (note how many of the defender’s bastions point outward.

The first such line of defenses was often started from the fortified camps and was built effectively out of range of the fort; this was the ‘first parallel.’ Now, by the time this project was well along, both the defenders and the attackers could do some basic calculations. They both might have a sense of how quickly reinforcements could arrive and if they were likely to be large enough to give battle or lift the siege. Both sides also know how long their supplies will last and can probably have a decent guess of how long their opponent’s supplies will last. And at this point, they can both calculate fairly well how many meters of earthwork and trench the attackers can dig per day and how many they need to dig to complete their siege operations.

But – and this is important – no one wants this siege to come down to its conclusive, final assault. The attackers don’t want this: every day their siege army sits out here, it is eating money such that each day wasted besieging this town limits what this army can accomplish overall before supplies and money run out. Moreover, the actual assault is likely to incur very high losses on the attackers, because even with a breach, they have to cross all of that open, fire-swept ground and then force the breach in close combat – and the defenders will know exactly where they are going days in advance. On the other hand, if the defenders make the attackers go through all of that effort, danger and death and lose – and they must assume the attackers wouldn’t have laid siege if they weren’t confident they had enough men to take the breach when it comes to it – then the city would be looted, its populace raped or massacred as the attackers vent their rage on the city. This was, at the time, considered the normal result of holding out, to the point that it seems to have been general practice that it was appropriate to wait something like three days before beginning the process of getting control of an army that had breached a city in this fashion.2

So once the attacker has completed that first parallel, he is going to send a message to the garrison, announcing that he has done so and offering them the chance to surrender. The standard terms for such surrender was called the ‘honors of war‘ – normally the defenders would be permitted to march out, with its flags flying, bayonets fixed, matches (for their matchlock muskets) lot on both ends and ‘ball in mouth’ (that is, a musketball held in their mouth ready to be swiftly loaded as protection against treachery). The defeated army was generally required to turn over its arms, but generally allowed to march back to their own territory. And finally, a town that surrenders like this might have to pay a ransom, but ought to be immune from pillage. To modern readers, these sorts of rituals seem quaint, like having a battle break for tea in the midafternoon (a thing that, to be clear, did not happen), but in fact these were fairly hard-nosed considerations: generous surrender terms aimed to induce a garrison or town to surrender and so spare the attacker both the time but also the blood of storming the place, because after all the goal here isn’t to destroy the enemy garrison but to get control of the town.

One that first parallel is completed, assuming the defender, upon doing the math, doesn’t decide the matter is hopeless and just give up, the attacker now proceeds to begin digging his works forward to set up advanced firing positions (most of the digging was done at night when enemy artillery couldn’t accurately fire at the work parties); there were typically three parallels, the first two providing protection to the digging works and the third parallel, dug into the glacis, providing the firing position directly into the structure of the fort.3 Once the cannon were emplaced, the attacker might again report that he was prepared to begin firing but if the defenders would just surrender, we could all skip that nasty business. If not, the attacker’s cannon – now secured in their own trace italienne-style firing positions – would start working on the enemy fortifications in a gunnery duel (since the defender’s guns are firing back). Progress here would mean both demolishing ravelins and other outworks which shielded the main curtain wall, as well as storming assaults on various outworks to take them and deny them to the enemy (Americans may note effectively all of these steps in the Siege of Yorktown (1781), albeit on fast-forward since this was a siege of a fairly small town defended merely by field fortifications rather than a large trace italienne fortress complex). The final option to surrender came once the curtain wall had been breached (there was little use in trying to ‘rush’ the breach since it would take days to produce so everyone knew where the final fight would be). The defender then had the terrible choice of either surrendering or risking the terrible slaughter that would follow if the breach was forced (as it was very likely to be).

This whole process could take a really long time and it involved a lot of digging, but it was almost mechanistic in its push towards success. In a sense, those demands forced European states to develop the sort of state capacity that had been the norm under Rome or in China in order to run this new version of the ‘big army’ siege playbook which demanded such tremendous amounts of work. As I’ve noted elsewhere, sieges of these sorts against large trace italienne complexes could last a long time; Parker (Military Revolution, 13) notes the siege of Breda in 1624 lasted nine months and was fairly short while the siege at Ostend in Flanders in 1601 lasted three years and was fairly long, to give a sense of the range.

British Shells and French Polygons

Naturally while these siege methods worked, they took a long time, cost a lot of money and thus limited the potential scope of military operations, so as you might imagine there were also efforts technologically to try to develop weapons that would be more effective against trace italienne style forts. While artillery usage improved tremendously from 1500 to 1700, it took a surprisingly long amount of time for those weapons to appear, but they did eventually appear, in the form of effective explosive shells.

Of course the basic idea, taking an iron shell, filling it with something that explodes and flinging it at the enemy, wasn’t new. But making exploding shells actually work demanded a lot of math. The physical technology was shrapnel shell: a shell designed to explode into small, lethal high speed fragments (often including not merely the metal shell the explosive charge was in, but small metal balls placed within it); the first of these was developed in 1784 by Henry Shrapnel (1761-1842), whose name eventually gave us the word shrapnel with its meaning. A shrapnel shell that burst directly over a bastion could easily kill most or all of the soldiers manning the cannon there, allowing an attacker to ‘clear’ the fortifications from long range. The potential of this kind of attack was recognized fairly quickly, but making it work took some doing. These exploding shells, after all, are the ‘bombs bursting in mid-air’ which nevertheless failed to take Fort McHenry out of commission (though at the same time, imagine what it means that the British fleet – and the American defenders! – considered it plausible that a single nights bombardment might knock out the small, trace italienne-style fort).

Via Wikipedia, the inside of a casemate, in this case a mid-19th century casemate at Fort Knox (not the one you are thinking of), Maine.

The problem is that the shell needs to explode at just the right moment in its arc to send its shrapnel raining down at the defenders. That means both that the shot needs to be accurate – the defenders need to be in its path! – but also that the fuse needs to be precisely timed to that very moment. Achieving that required both much better cannon and far more precise ballistics. Luckily for European gunners (and unluckily for their targets), the scientific and industrial revolutions in Europe were in the process of furnishing both and by the second quarter of the 1800s, European gunners had gotten very good at this trick. One may, for instance, consider Tonio Andrade (op. cit.)’s description of British gunners tearing apart Qing-dynasty Chinese forts in the First Opium War (1839-42), forts that were designed to resist cannon to see the effectiveness of European gunners against open-topped forts using shrapnel shells by the 1840s.

Via Wikipedia, three tiers of artillery casemates from a mid-19th century fort, Fort Point, in San Francisco.

The response to these shells was what is sometimes called the ‘polygonal fort.’ These forts come in substantially simpler shapes than the almost baroque designs of the trace italienne, but their major innovation was in placing the defensive guns in multi-story stone or brick casemates, big vaulted chambers built into the walls of the fort. The basic principles of these new kinds of forts were proposed by Marc René and Lazare Carnot in France, with the first fully developed examples of the type emerging in the first decades of the 1800s. The industrial revolution had made cannon cheaper, so the possibility of mounting multiple stories of cannon, one atop the other, to generate crushing firepower was possible (and of course shrapnel and canister shot meant that such forts could develop enough firepower that they couldn’t be approached). For Americans, the most famous example of such a fort is Fort Sumter, but many civil war era permanent forts were of this style. To control the approach, the ditch – which was preserved – was defended by caponiers, covered shooting positions that projected into the moat with firing positions to enable the defenders to fire down the base of the walls. By being covered, the caponiers were shielded from shrapnel shell and by virtue of being entirely submerged down into the ditch, they avoided simply being battered apart by enemy direct-fire artillery.

Via WIkipedia, a diagram of a caponier, projecting out into the ditch in order to provide that enfilade fire.

That said, the polygonal style of fort itself wouldn’t last very long. The improved machining of the industrial revolution had made it possible to rifle cannon the same way that muskets could be rifled; as with muskets, such rifling would allow for much greater accuracy and power. During the American Civil War (1861-1865), gunners – particularly United States gunners, since the US Army, being generally on the offensive, tended to do most of the attacking forts apart from that first one and also had far better access to modern rifled cannon – demonstrated that these newer, larger and more powerful rifled siege guns could reduce brick and stone polygonal forts to rubble with startling speed. Fort Pulaski, in Georgia, a single-story (plus the rampart) polygonal fort, was breached in just 30 hours by US rifled cannon (James and Parrott rifles, in the event) under the command of Quincy A. Gilmore.

By contrast, earthworks – that is, field fortifications like trenches – held up much better under bombardment. Indeed, in the case of the famous Fort Sumter, after the US Navy blockade essentially reduced the by-then-Confederate held fort to rubble, Confederate gunners mounted their cannon in the ruins, essentially turning the shattered brick into earthworks which couldn’t also be flattened (though of course had this fort not been on an island, the attacker could have simply stormed it at this point). That said, it is important not to overstate this; field works in the civil war mostly remained breastworks (that is, built above ground) rather than trenches (though these were still used in sieges) and the emergence of a sort of proto-trench warfare had as much if not more to do with the decision by US commanders (particularly Grant) to remain in contact with Confederate armies in order to pressure them as it had to do with firepower.

Via Wikipedia, the shattered remains of Fort Sumter.

The other development in the offing which would be even more potentially fatal to large, obvious fortresses was the emergence of other explosives than gunpowder. While dynamite, patented in 1867, was generally too unstable to be used in exploding shells (it tended to explode in the barrel), but effective high explosive shells which could blast apart walls and earthworks (penetrating and then exploding) began to emerge in the 1880s, forcing new fortification designs, which we’ll discuss as the last part of this series next time. But not next week. Next week is Christmas Eve and I’m taking it off. So I’ll see y’all on the 31st.

1. A non-exhaustive list (some dates approximate): the Ottomans absorb Byzantium (1453), Serbia (1459), Morea (1460), Trebizond (1461), Bosnia (1463), Albania (1468) and Theodoro (1475). Muscovy absorbs Novgorod (1478), Tver (1485), and Pskov (1510). Spain absorbs Aragon (1479), Granada (1492), Navarre (1513) and parts of Burgundy (1482). France absorbs Burgundy (1482), Provence (1486), Auvergne (1527), and Brittany (1547). Austria absorbs Bohemia (1526) and splits Hungary with the Ottomans (1526). And Poland destroys the Teutonic Order (1521) before Poland and Lithuania absorb each other into the Polish-Lithuanian Commonwealth (1569). There’s also a lot of smaller-scale consolidation within the Holy Roman Empire, but untangling all of the lands and titles there is beyond my ken.
2. Needless to say this description of the realities of early modern European warfare should not be taken as in any way approving of it. As I’ve noted elsewhere, the warfare of this period was remarkable in its destructiveness.
3. The full three-parallel method comes relatively late and is codified by Sébastien le Marquis de Vauban (1633-1707), though some form of ‘dig forward until you get to the glacis, then blast them’ had been in use much longer