We’ve gone over this before, but we should also cover the objectives the attacker generally has in a siege. In practice, we want to think about assaults fitting into two categories: the raid and the siege, with these as distinct kinds of attack with different objectives. The earliest fortifications were likely to have been primarily meant to defend against raids rather than sieges as very early (Mesolithic or Neolithic) warfare seems, in as best we can tell with the very limited evidence, to have been primarily focused on using raids to force enemies to vacate territory (by making it too dangerous for them to inhabit by inflicting losses). Raids are typically all about surprise (in part because the aim of the raid, either to steal goods or inflict casualties, can be done without any intention to stick around), so fortifications designed to resist them do not need to stop the enemy, merely slow them down long enough so that they can be detected and a response made ready. […]

In contrast, the emergence of states focused on territorial control create a different set of strategic objectives which lead towards the siege as the offensive method of choice over the raid. States, with their need to control and administer territory (and the desire to get control of that territory with its farming population intact so that they can be forced to farm that land and then have their agricultural surplus extracted as taxes), aim to gain control of areas of agricultural production, in order to extract resources from them (both to enrich the elite and core of the state, but also to fund further military activity).

Thus, the goal in besieging a fortified settlement (be that, as would be likely in this early period, a fortified town or as later a castle) is generally to get control of the administrative center. Most of the economic activity prior to the industrial revolution is not in the city; rather the city’s value is that it is an economic and administrative hub. Controlling the city allows a state to control and extract from the countryside around the city, which is the real prize. Control here thus means setting up a stable civilian administration within the city which can in turn extract resources from the countryside; this may or may not require a permanent garrison of some sort, but it almost always requires the complete collapse of organized resistance in the city. Needless to say, setting up a stable civilian administration is not something one generally does by surprise, and so the siege has to aim for more durable control over the settlement. It also requires fairly complete control; if you control most of the town but, say, a group of defenders are still holding out in a citadel somewhere, that is going to make it very difficult to set up a stable administration which can extract resources.

Fortunately for potential defenders, a fortification system which can withstand a siege is almost always going to be sufficient to prevent a raid as well (because if you can’t beat it with months of preparatory work, you are certainly unlikely to be able to quickly and silently overcome it in just a few night hours except under extremely favorable conditions), though detection and observation are also very important in sieges. Nevertheless, we will actually see at various points fortification systems emerge from systems designed more to prevent the raid (or similar “surprise” assaults) rather than the siege (which is almost never delivered by surprise), so keeping both potential attacking methods in mind – the pounce-and-flee raid and the assault-and-stay siege – is going to be important.

As we are going to see, even fairly basic fortifications are going to mean that a siege attacker must either bring a large army to the target, or plan to stay at the target for a long time, or both. In a real sense, until very recently, this is what “conventional” agrarian armies were: siege delivery mechanisms. Operations in this context were mostly about resolving the difficult questions of how to get the siege (by which I mean the army that can execute the siege) to the fortified settlement (and administrative center) being targeted. Because siege-capable armies are either big or intend to stick around (or both), surprise is out of the window for these kinds of assaults, which in turn raises the possibility of being forced into a battle, either on the approach to the target or once you have laid siege to it.

It is that fact which then leads to all of the many considerations for how to win a battle, some of which we have discussed elsewhere. I do not want to get drawn off into the question of winning battles, but I do want to note here that the battle is, in this equation, a “second order” concern: merely an event which enables (or prohibits) a siege. As we’ll see, sieges are quite unpleasant things, so if a defender can not have a siege by virtue of a battle, it almost always makes sense to try that (there are some exceptions, but as a rule one does not submit to a siege if there are other choices), but the key thing here is that battles are fundamentally secondary in importance to the siege: the goal of the battle is merely to enable or prevent the siege. The siege, and the capture or non-capture of the town (with its role as an administrative center for the agricultural hinterland around it) is what matters.

Bret Devereaux, “Collections: Fortification, Part I: The Besieger’s Playbook”, A Collection of Unmitigated Pedantry, 2021-10-29.