From a modern, civilian perspective, sieges may seem kind of stupid. Why park an army outside a fortified castle or city for weeks, months, even years? Isn’t there a better way to do things?
To understand siege warfare, you have to think about that larger question I posed last week: what is the purpose of your military undertaking? Hunter-gatherers who want to benefit from resource-rich areas have to kill or drive off the people currently occupying them, but once agriculture enters the picture, that doesn’t work nearly so well. If you kill or drive off all the peasantry, who’s going to work the land? It’s unlikely that you have enough surplus population to straight-up replace them, and even if you do, your own peasants may be less than enthusiastic about leaving their neighbors and ancestral lands behind to go till fields in a place whose soil and weather conditions are unknown to them. Better by far — if you can arrange it — to subjugate the people who are already there.
Fortunately for you, the local government has already done that! Places like castles and cities are nexuses, used to control the surrounding countryside. If you can take one over, then you get all the benefits of the ready-made administrative center. It’s like a joint lock in interpersonal combat: you don’t have to control the whole body; you just need to control a key piece that reduces the other guy’s ability to hit you. And if you don’t take that nexus — if you move onward with your army toward some other target, without first neutralizing the one behind you — then you leave yourself at serious risk of being hit in the rear by an enemy force. Remember last week, when I talked about armies raiding each others’ baggage trains? Sallying out from the castle with a small force to burn the invader’s grain wagons and steal their payroll chests is a great way to stop that invasion in its tracks.
But because these locations are so valuable, they’re also protected. I’ll talk about fortifications in more detail someday (because their actual function is sometimes misunderstood); for now, suffice it to say that the fortifications are there, preventing a swift in-and-out strike. You have to overcome those defenses, and depending on the relative capabilities of defender and attacker, that is not a quick process.
So how do you do it?
Although media likes to show cities and castles falling to dramatic assaults, in many cases, a siege looks more like a game of chicken: can the target hold out long enough for reinforcements to arrive? The attacking army really doesn’t want that to happen; it will mean they’re caught between a hammer and an anvil. They’ll usually flee rather than let that happen. But in the broader context of war, reinforcements may be tied up elsewhere, fighting field battles or holding out against their own besiegers. And the longer the target holds out, the worse things get for them, not just before the surrender but probably after it as well. Depending on how tight the siege is, the target may or may not even have any sense of whether help is on the way.
That question of tightness is a key one. There really are sieges that went on for years; when that happens, it means the attacker hasn’t been able to choke the target off from outside interaction. Usually this happens because of failure, through lack of personnel or lack of skill, to control a seaport or river (or in more modern times, a railroad), thereby allowing the target to continue importing food, supplies, and even new forces. Fantasy and science fiction writers might need to consider things like air access and teleportation as well. Even if that level of access is blocked, if the people inside can communicate with the outside, they’re far more able to warn others about their situation and calculate whether reinforcements will arrive in time. From the perspective of the attacker, it’s far better to isolate them completely.
And — just as in interrogation — you can often accomplish a lot by offering (relative) mercy: if they surrender now, you’ll let them off easy. The longer they wait, the worse things will get for them. Quite a few groups, including the Assyrians, the Romans, and Genghis Khan’s Mongol army were notorious for the brutality with which they treated targets that resisted, in order to encourage others to give up without a fight. Warfare has a psychological dimension as well as a physical one; the bombardment of the walls is as much to wear away at the sanity and confidence of the defenders as it is to overcome the defenses themselves. If the besieged population is literate, you can toss in propaganda to sap their will; if they aren’t, you can toss in things like the corpses of their dead friends. (But be careful: that might just harden their resistance.)
This doesn’t mean the dramatic stuff never happened. But flinging ladders against the walls and climbing up in the hopes that nobody stabs you at the top? Not a good idea unless the wall is so poorly defended that you can be pretty sure of getting a foothold there before resistance arrives. You do that by first clearing the wall, possibly by using a siege tower to fire down on the defenders (much more effective than just hoping your guys can charge off the tower in good order). If you have an extremely well-organized army, you can even build a mole, a ramp that lets your forces literally run up to the top of the wall and fight from there.
Or you can try to break the walls. This is usually the actual purpose of sapping, i.e. digging under the wall; it’s not a useful way to deliver troops inside the fortifications, but it can weaken the foundations enough for bombardment above to knock things over. How likely the latter is depends in part on what you’re bombarding with. Rams can do it, given enough time, though they’re better used against towers — yes, towers, not gates! The gates are very strong, while the towers are where a lot of defenders are shooting from. Catapults and the like perform better, but this kind of thing really comes into its own with gunpowder: in 1494, the fortification of Mordano fell in less than a day in the face of vastly improved cannonry.
Even if you do get a wall to collapse, though, you’re not necessarily charging straight into the city. After all, why risk a fight through a bottleneck if you don’t need to? Go back to the psychological warfare playbook: point out to the defenders that they are now extremely vulnerable, and give them one last chance to surrender.
And then there’s always the possibility of treachery. Even if some defenders are determined to hold out, a faction who don’t think reinforcements will arrive in time, or who have succumbed to promises of reward from the attackers, might betray the target from within. Like all-out assault, I suspect this shows up more often in stories than in reality . . . but it does make for some good fiction.
All of this, though, is only one part of the story. Sieges are going to get two essays, because unlike our other forms of battle, they last long enough for it to be worth talking about what life is like under siege. (Spoiler: absolutely horrific.)