While fortifications obviously had existed a long time, when we talk about castles, what we really mean is a kind of fortified private residence which also served as a military base. This form of fortification really only becomes prominent (as distinct from older walled towns and cities) in 9th century, in part because the collapse of central authority (due in turn to the fragmentation of the Carolingian Empire) led to local notables fortifying their private residences. This process was, unsurprisingly, particularly rapid and pronounced in the borderlands of the various Carolingian splinter kingdoms (where there were peer threats from the other splinters) and in areas substantially exposed to Scandinavian (read: Viking) raiding. And so functionally, a castle is a fortified house, though of course large castles could encompass many other functions. In particular, the breakdown of central authority meant that these local aristocrats also represented much of the local government and administration, which they ran not through a civil bureaucracy but through their own households and so in consequence their house (broadly construed) was also the local administrative center.

Now, we can engage here in a bit of a relatable thought experiment: how extensively do your fortify your house (or apartment)? I’ll bet the answer is actually not “none” – chances are your front door locks and your windows are designed to be difficult to open from the outside. But how extensive those protections are vary by a number of factors: homes in high crime areas might be made more resistant (multiple deadbolts, solid exterior doors rather than fancy glass-pane doors, possibly even barred windows at ground level). Lots of neighbors can lower the level of threat for a break-in, as can raw obscurity (as in a house well out into the country). Houses with lots of very valuable things in them might invest in fancy security systems, or at least thief deterring signs announcing fancy security systems. And of course the owner’s ability to actually afford more security is a factor. In short, home defenses respond to local conditions aiming not for absolute security, but for a balance of security and cost: in safe places, home owners “consume” that security by investing less heavily in it, while homeowners who feel less safety invest more in achieving that balance, in as much as their resources allow. And so the amount of security for a house is not a universal standard but a complicated function of the local danger, the resources available and the individual home owner’s risk tolerance. Crucially, almost no one aims for absolute home security.

And I go through this thought process because in their own way the same concerns dictate how castles – or indeed, any fortification – is constructed, albeit of course a fortified house that aims to hold off small armies rather than thieves is going to have quite a bit more in the way of defenses than your average house. No fortification is ever designed to be absolutely impenetrable (or perhaps most correctly put, no wise fortress designer ever aims at absolute impenetrability; surely some foolish ones have tried). This is a fundamental mistake in assessing fortifications that gets made very often: concluding that because no fortification can be built to withstand every assault, that fortification itself is useless; but withstanding every assault is not the goal. The goal is not to absolutely prohibit every attack but merely to raise the cost of an attack above either a potential enemy’s willingness to invest (so they don’t bother) or above their ability to afford (so the attack is attempted and fails) and because all of this is very expensive the aim is often a sort of minimum acceptable margin of security against an “expected threat” (which might, mind you, still be a lot of security, especially if the “expected threat” is very high). This is true of the castle itself, if for no other reason than that resources are scarce and there are always other concerns competing for them, but also for every component of its defenses: individual towers, gates and walls are not designed to be impenetrable, merely difficult enough.

This is particularly true in castle design because the individuals building these castles often faced fairly sharp limitations in the resources at their disposal. Castles as a style of fortification emerge in a context of political fragmentation, in particular the collapse of the Carolingian Empire, which left even the notional large kingdoms (like the kingdom of France) internally fragmented. Castles were largely being built not by kings but by counts and dukes who held substantial landholdings but nothing like the resources of Charlemagne or Louis the Pious, much less the Romans or Assyrians. Moreover, the long economic and demographic upswing of the Middle Ages was only just beginning to gain momentum; the great cities of the Roman world had shrunk away and the total level of economic production declined, so the sum resources available to these rulers were lower. Finally, the loss of the late Roman bureaucracy (replaced by these fragmented realms running on an economic system best termed “manorialism”) meant that the political authorities (the nobility) often couldn’t even get a hold of a very large portion of the available economic production they did have. Consequently, castle construction is all about producing what security you can with as little labor, money and resources as possible (this is always true of any fortification, mind you, merely that in this period the resource constraints are much tighter).

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