The earliest castle designs we see in Europe during the Middle Ages are wooden “motte and bailey” castles which emerge first during the 10th century and make their way to Britain after 1066. In the initial basic form, the core structure (the “keep”, which is typically the fortified house itself) is placed on a motte, a hill (usually artificial) with a flattened top. The keep itself is constructed as a tall, wooden tower, with the height offering advantages both as a fighting position and for observation of the surrounding area. The motte is then enclosed by a wooden palisade (often two, one at the base of the motte and another at the crest) and surrounded by a ditch (the moat, which would be filled with water if it could be connected to a river or stream, but could also be left “dry” and still serve its purpose), the dirt of which was used to build up the motte in the first place.

But as noted, the personal manor home of a significant noble (the rank in this case is often a “castellan”, literally the keeper of a castle, so entrusted by one of the more powerful nobles who holds sway over a larger territory; the castellan has the job of holding the castle and administering the countryside around it) is also an administrative center, managing the extraction of agricultural surplus from the countryside and also a military base, housing the physical infrastructure for that noble’s retinue, which again is the fundamental building block of larger armies. Which means that it is going to need more structures to house those functions: stables for horses, storehouses for food, possibly food processing facilities (bakeries, mills) and living space both for retainers (be they administrators or military retainers) and for the small army of servants such a household expects. Those structures (to the degree they can’t exist in the keep) are put in the bailey, a wider enclosed part of the settlement constructed at the base of the motte. As with the motte, the bailey is typically enclosed only by a wooden palisade; naturally that means the most valuable things (the physical treasury, the lord’s family) go in the keep on the motte, while the more space-demanding but less valuable things go in the bailey. There is a lot of room for variation in this basic type, but for now the simple version will serve.

The resulting fortification seems almost paradoxically vulnerable. The bailey, after all, is protected only by a ditch and a wooden palisade which a determined work-party could breach with just iron axes and an afternoon to kill. The core defensive motte with its keep adds perhaps only one more palisade and a steep climb. But in fact, these relatively modest defenses have greatly increased the cost of attacking this settlement. The motte and bailey castle, at least in its early wooden form, won’t stand up to a determined assault by a large and well-coordinated enemy, but that isn’t its purpose. Instead, the purpose of the motte and bailey castle is to raise the cost of an assault such that a potential opponent must bring a significant force and make a careful, well-planned assault; this the motte and bailey accomplishes quite well, which explains the long durability of the basic design, with stone versions of the motte and bailey persisting into the 15th century.

The quick mounted raid is now impossible; precisely because it will take a solid afternoon to breach the defenses, there is little hope of surprising the defenders. At the same time, the ditches will make any such work party vulnerable to missile fire (arrows, yes, but also javelins or just large rocks) from the palisade. And most of all, taking the place now demands you coordinate a work party, with some of your attackers splitting up to suppress the defenders, some making sure to block the exits so the defenders don’t rush out and attack your work party directly, and still more of your attackers in the work party itself. These very basic defenses have suddenly taken you from a position where a bit of surprise and rough numerical parity was enough to contemplate an assault to a position where you need several times as many attackers (for each of those divisions needs to be large enough to confidently win against the defenders if assailed).

Perhaps most importantly, the basic structure of this defense demands that you do this multiple times in sequence. We’ve already discussed the value of defense-in-depth, but in brief, every attack is at its strongest in the moment after it jumps off: everyone is alive, in the right positions, at the right time, coordinated and at least in theory clear on their objectives. Every movement and action beyond this point diminishes the power of the effort as coordination breaks down, attackers are killed and things break; this is what Clausewitz terms (drink!) friction – the unpredictable interaction of probabilities takes their toll on any plan, no matter how carefully designed. This is, by the by, more true in real warfare, where coordination is limited by communications technology, than it is in film or video games, where armies appear to mostly communicate by some form of instantaneous telepathy (it is amazing just how many clever sounding movie or game assault plans fall apart once you imagine trying to coordinate them with nothing more than shouting, or even a radio). As more and more things turn out unexpectedly or have to be improvised, the plan slowly shakes apart until eventually all of the momentum is lost.

The basic structure of a motte and bailey castle exploits this feature of warfare, forcing an attacker to overcome a series of obstacles in sequence, all while in contact with the enemy. Recall that this is a defense which really doesn’t envisage enemy artillery (because armies with lots of effective siege artillery were not common in the often small-scale warfare of the period; that’s not to say they didn’t exist, but if your motte and bailey castle forces the enemy to only attack with a big, expensive army that can build catapults, it has done its job, not the least because most possible enemies won’t have that capability at all), so an attacker is going to have to breach each layer in sequence while in contact with the defense and to pierce them all more or less “in one go”. Consequently, taking the castle by storm means crossing (and probably filling in) at least one deep ditch, breaching a palisade under fire, then moving up a steep hill under fire, then breaching another palisade, at the end of all of which, the attacker must arrive at the keep with enough force and cohesion to take it. All of that is going to take a substantial attack and a lot of coordination and most potential attackers, the defender may hope, will lack either the resources or the determination to go through so much effort, especially as they are likely to have to do it multiple times: being entirely wooden, motte and bailey castles were fairly cheap and so a large territory could have quite a lot of them (note on the Bayeux Tapestry how William has to take several such castles in order to capture Conan II of Britanny). Each motte and bailey castle thus raises the cost of trying to seize control of the territory; collectively they make that cost prohibitive.

Of course our principle of “antagonistic co-evolution” is not done and the vulnerabilities of a wooden motte and bailey castle are fairly clear and easy to exploit. For one, the wooden palisade is mostly a blocking element, rather than a fighting position; attackers that reach the wall can actually use it as cover while tearing it down or setting it on fire. The entire setup, being made of wood, is vulnerable to fire but also to any kind of even-quite-modest catapult. And quite naturally, any military leader (which is to say, the military aristocracy which was emerging at the very same time as these castles) is going to want to build the kind of capabilities which will allow for successful castle assaults because, as we’ve already noted, castles function more or less as the “nails” on the map which hold down the canvas of revenue extraction and military power.

Which in turn means evolving castle design to resist the methods by which a motte and bailey castle might fall. The most immediate change is in building material: wooden walls can only be so high, so thick and so resistant to fire. Stone, though far more expensive, offers advantages on all three fronts. And so, already in the late 10th century, we start to see stone keeps and gatehouses (supporting still wooden palisades); full stone castles would soon follow.

As an aside, one solution to this problem which doesn’t much appear in the Middle Ages but was very well-used in Iron Age Europe was what the Romans called the murus Gallicus, a hybrid wood-and-stone wall system. Gallic hillforts (called oppida) were built on hills, as the name suggests; their outer walls could be built by using earth fill to construct what was essentially a retaining wall, faced in stone, with transverse reinforcing wood beams every few feet. That created, in turn, a vertical stone surface, supported by the hillside itself, on which could be additionally built a wooden palisade for added height. The result was a very formidable fortification, assuming one had the hill to work with initially. You couldn’t knock it over or really undermine it effectively and the stone face was nearly vertical; the height of the hill meant that effective escalade meant coming up with a mole, tower or ladder taller than the hill (a thing, naturally, that the Romans ended up doing). That this style of fortification didn’t really reemerge in the Middle Ages speaks to the degree of path dependence in fortification design. Because fortification design tends to be evolutionary, it is possible in similar conditions to get very different responses as different designers try to meet the same threats by modifying different preexisting systems of fortification.

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