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).

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.

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.

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.

Bret Devereaux, “Collections: Fortification, Part IV: French Guns and Italian Lines”, A Collection of Unmitigated Pedantry, 2021-12-17.