… the expected threat is going to shape the calculation of what margin of security is acceptable, which brings us back to our besieger’s playbook. You may recall when we looked at the Assyrian siege toolkit, that many of the most effective techniques assumed a large, well-coordinated army which could dispose of a lot of labor (from the soldiers) on many different projects at once while also having enough troops ready to fight to keep the enemy bottled up and enough logistic support to keep the army in the field for however long all of that took. In short, this is a playbook that strong, well-organized states (with strong, well-organized armies) are going to excel at. But, as we’ve just noted, the castle emerges in the context of fragmentation which produces a lot of little polities (it would be premature to call them states) with generally quite limited administrative and military capacity; the “big army” siege playbook which demands a lot of coordination, labor and expertise is, for the most part, out of reach.

Clifford Rogers has already laid out a pretty lay-person accessible account of the medieval siege playbook (in Soldiers’ Lives Through History: The Middle Ages (2007), 111-143; the book is pricey, so consider your local library), so I won’t re-invent the wheel here but merely note some general features. Rogers distinguishes between hasty assaults using mostly ladders launched as soon as possible as a gamble with a small number of troops to try to avoid a long siege, and deliberate assaults made after considerable preparation, often using towers, sapping, moveable shelters designed to resist arrow fire and possibly even catapults. We’ve already discussed hasty assaults here, so let’s focus on deliberate assaults.

While sapping (tunneling under and collapsing fortifications) remained in use, apart from filling in ditches, the mole-and-ramp style assaults of the ancient world are far less common, precisely because most armies (due to the aforementioned fragmentation combined with the increasing importance in warfare of a fairly small mounted elite) lacked both the organizational capacity and the raw numbers to do them. The nature of these armies as retinues of retinues also made coordination between army elements difficult. The Siege of Antioch (1097-8) [during] the First Crusade is instructive; though the siege lasted nine months, the crusaders struggled to even effectively blockade the city until a shipment of siege materials (lumber, mostly) arrived in March of 1098 (five months after the beginning of the siege). Meanwhile, coordinating so that part of the army guarded the exits of the city (to prevent raids by the garrison) while the other part of the army foraged supplies had proved mostly too difficult, leading to bitter supply shortages among the crusaders. Even with materials delivered to them, the crusaders used them to build a pair of fortified towers blocking exits from the city, rather than the sort of elaborate sapping and ramps; the city was taken not by assault but by treachery – a very common outcome to a siege! – when Bohemond of Taranto bribed a guard within the city to let the crusaders sneak a small force in. All of this despite the fact that the crusader army was uncommonly large by medieval European standards, numbering perhaps 45,000.

Crucially, in both hasty and deliberate assaults, the emphasis for the small army toolkit tends to be on escalade (going over the walls) using ladders or moveable wooden towers, rather than the complex systems of earthworks favored by the “big army” siege system or breaching – a task which medieval (or ancient!) artillery was generally not capable of. The latter, of course, is a much more certain method of assault – give a Roman army a few months and almost any fortress could be taken with near certainty – but it was a much more demanding method in terms of the required labor and coordination. Thwarting escalade is mostly a question of the height of defenses (because a taller wall requires a taller ladder, tower or ramp) and good fields of fire for the defenders (particularly the ability to fire at attackers directly up against the wall, since that’s where the ladders are likely to be).

The other major threat to castle walls (apart from the ever-present threat of sapping) was catapults, but I want to deal with those next time for reasons that I suspect will make sense then. For now it is worth simply noting that catapults, even the mighty trebuchets of the 14th century were generally used to degrade defenses (smashing towers, destroying crenellation, damaging gatehouses) rather than to produce breaches. They could in some cases do that, but only with tremendous effort and a lot of time (and sometimes not even then). Consequently, for most castles the greatest threat remained escalade, followed by treachery or starvation, followed by sapping, followed by artillery.

Bret Devereaux, “Collections: Fortification, Part III: Castling”, A Collection of Unmitigated Pedantry, 2021-12-10.