As we move to stone construction and especially full stone construction (which we’ll define as the point when at least one complete curtain wall – don’t worry, we’ll define that in a second – is in stone) in the 12th century, we’re beginning to contemplate a different kind of defense. The wooden motte and bailey, as we’ve seen, mostly served to resist both raids and “hasty” assaults, thus forcing less coordinated or numerous attackers to set in to starve the castle out or go home. But stone walls are a much larger investment in time and resources; they also require a fair bit more careful design in order to be structurally sound. For all of that expense, the builder wants quite a bit of a security, and in the design of stone castles it is hard not to notice increasing attention towards resisting a deliberate assault; stone castles of the 12th century and beyond are increasingly being designed to stand up to the best that the “small army” playbook can throw at them. Of course it is no accident that this is coming at the same time that medieval European population and wealth is beginning to increase more rapidly, leaving political authorities (read: the high nobility) with both the resources for impressive new castles (although generally the number of castles falls during this period – fewer, stronger castles) and at the same time with more resources to invest in the expertise of siegecraft (meaning that an attacker is more likely to have fancy tools like towers, catapults and better coordination to use them).

To talk about how these designs work, we need to clear some terminology. The (typically thin) wall that runs the circuit of the castle and encloses the bailey is called a “curtain wall“. In stone castles, there may be multiple curtain walls, arranged concentrically (a design that seems to emerge in the Near East and makes its way to Europe in the 13th century via the crusades); the outermost complete circuit (the primary wall, as it were) is called the enceinte. Increasingly, the keep in stone castles is moved into the bailey (that is, it sits at the center of the castle rather than off to one side), although of course stone versions of motte and bailey designs exist. In some castle design systems, with stone the keep itself drops away, since the stone walls and towers often provided themselves enough space to house the necessary peacetime functions; in Germany there often was no keep (that is, no core structure that contained the core of the fortified house), but there often was a bergfriede, a smaller but still tall “fighting tower” to serve the tactical role of the keep (an elevated, core position of last-resort in a defense-in-depth arrangement) without the peacetime role.

While the wooden palisade curtain walls of earlier motte and bailey castles often lacked many defensive features (though sometimes you’d have towers and gatehouses to provide fighting positions around the gates), stone castles tend to have lots of projecting towers which stick out from the curtain wall. The value of projecting towers is that soldiers up on those towers have clear lines of fire running down the walls, allowing them to target enemies at the base of the curtain wall (the term for this sort of fire is “enfilade” fire – when you are being hit in the side). Clearly what is being envisaged here is the ability to engage enemies doing things like undermining the base of walls or setting up ladders or other scaling devices.

The curtain walls themselves also become fighting positions. Whether on a tower or on the wall itself, the term for the fighting position at the top is a “battlement”. Battlements often have a jagged “tooth” pattern of gaps to provide firing positions; the term for the overall system is crenellation; the areas which have stone are merlons, while the gaps to fire through are crenals. The walkway behind both atop the wall is the chemin de ronde, allure or “wall-walk”. One problem with using the walls themselves as fighting positions is that it is very hard to engage enemies directly beneath the wall or along it without leaning out beyond the protection of the wall and exposing yourself to enemy fire. The older solution to this were wooden, shed-like projections from the wall called “hoarding”; these were temporary, built when a siege was expected. During the crusades, European armies encountered Near Eastern fortification design which instead used stone overhangs (with the merlons on the outside) with gaps through which one might fire (or just drop things) directly down at the base of the wall; these are called machicolations and were swiftly adopted to replace hoardings, since machicolations were safer from both literal fire (wood burns, stone does not) and catapult fire, and also permanent. All of this work on the walls and the towers is designed to allow a small number of defenders to exchange fire effectively with a large number of attackers, and in so doing to keep those attackers from being able to “set up shop” beneath the walls.

[I]t is worth noting something about the amount of fire being developed by these projecting towers: the goal is to prevent the enemy operating safely at the wall’s base, not to prohibit approaches to the wall. These defenses simply aren’t designed to support that much fire, which makes sense: castle garrisons were generally quite small, often dozens or a few hundred men. While Hollywood loves sieges where all of the walls of the castle are lined with soldiers multiple ranks deep, more often the problem for the defender was having enough soldiers just to watch the whole perimeter around the clock (recall the example at Antioch: Bohemond only needs one traitor to access Antioch because one of its defensive towers was regularly defended by only one guy at night). It is actually not hard to see that merely by looking at the battlements: notice in the images here so far often how spaced out the merlons of the crenellation are. The idea here isn’t maximizing fire for a given length of wall but protecting a relatively small number of combatants on the wall. As we’ll see, that is a significant design choice: castle design assumes the enemy will reach the walls and aims to prevent escalade once they are there; later in this series we’ll see defenses designed to prohibit effective approach itself.

As with the simpler motte and bailey, stone castles often employ a system of defense in depth to raise the cost of an attack. At minimum, generally, that system consists of a moat (either wet or dry), the main curtain walls (with their towers and gatehouses) and then a central keep. Larger castles, especially in the 13th century and beyond, adopting cues from castle design in the Levant (via the crusades) employed multiple concentric rings of walls. Generally these were set up so that the central ring was taller, either by dint of terrain (as with a castle set on a hill) or by building taller walls, than the outer ring. The idea here seems not to be stacking fire on approaching enemies, but ensuring that the inner ring could dominate the outer ring if the latter fell to attackers; defenders could fire down on attackers who would lack cover (since the merlons of the outer ring would face the other way). As an aside, the concern to be firing down is less about the energy imparted by a falling arrow (though this is more meaningful with javelins or thrown rocks) and more about a firing position that denies enemies cover by shooting down at them (think about attackers, for instance, crossing a dry moat – if your wall is the right height and the edges of the moat are carefully angled, you can set up a situation where the ditch never actually offers the attackers any usable cover, but you need to be high up to do it!).

Speaking of the moat, this is a common defensive element (essentially just a big ditch!) which often gets left out of pop culture depictions of castles and siege warfare, but it accomplishes so many things at such a low cost premium. Even assuming the moat is “dry”! For attackers on foot (say, with ladders) looking to approach the wall, the moat is an obstacle that slows them down without potentially providing any additional cover (it is also likely to disorder an attack). For sappers (attackers looking to tunnel under the walls and then collapse the tunnel to generate a breach), the depth of the ditch forces them to dig deeper, which in turn raises the demands in both labor and engineering to dig their tunnel. For any attack with siege engines (towers, rams, or covered protective housings made so that the wall can be approached safely), the moat is an obstruction that has to be filled in before those engines can move forward – a task which in turn broadcasts the intended route well in advance, giving the defenders a lot of time to prepare.

Well-built stone castles of this sort were stunningly resistant to assault, even with relatively small garrisons (dozens or a few hundred, not thousands). That said, building them was very expensive; maintaining them wasn’t cheap either. For both castles and fortified cities, one ubiquitous element in warfare of the period (and in the ancient period too, by the by) was the rush when war was in the offing to repair castle and town walls, dig out the moat and to clear buildings that during peace had been built int he firing lines of the castle or city walls.

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