How a Siege Actually Works

Picture a walled medieval city. The walls are thirty feet high and ten feet thick. Behind them are wells, granaries, livestock, and a few thousand people. Outside them is an army that wants in. In the movies, the army builds a giant catapult, smashes the gate, and pours through. That happens sometimes. But most sieges were not won that way. Most sieges were won by waiting.

A siege is the military operation of surrounding a fortified place and cutting it off from the outside world until the people inside cannot keep fighting. The word for this surrounding is investment: the attacking army builds a ring of camps, ditches, and watchtowers around the city so nothing can go in and nothing can come out. No grain wagons. No reinforcements. No messengers. Once a city is fully invested, the clock starts.

The clock is the real weapon. Inside the walls, food is finite. So is firewood, so is fodder for animals, and so is the patience of hungry people. A city of ten thousand might have six months of grain stored if its rulers planned well, or six weeks if they did not. The besieging army's job is to outlast that supply. This is why sieges were often less about brave assaults and more about boring math: how many mouths inside, how much grain per mouth per day, how many days until the math runs out.

But the attackers had a clock of their own. An army camped in one place for months gets sick. Dysentery and typhus killed more besiegers than arrows ever did, because thousands of soldiers in tents next to their own waste is a disease factory. The attackers also had to be fed, and feeding an army in enemy territory is harder than feeding a city sitting on its own stockpile. Winter could end a siege. So could news that another army was marching to relieve the city — meaning, to attack the besiegers from behind and break the ring.

So a siege was really two clocks running against each other. The defenders were betting that hunger inside the walls would arrive after disease, winter, or a relief army arrived outside them. The attackers were betting the opposite. Everything else — the catapults, the tunnels dug under the walls, the towers rolled up to the battlements — was mostly there to speed up the defenders' clock or to make the defenders agree to surrender on terms before either clock ran out.

Surrender on terms mattered enormously. A city that opened its gates after a short siege was usually spared. A city that forced the attackers to storm the walls, costing them lives, was often sacked: looted, burned, its people killed or enslaved. Both sides knew this, which meant the threat of an assault was sometimes more useful than the assault itself. A commander who could convince the defenders that the walls would fall might get the gates opened without a fight.

This is why the question "how long can you hold out?" mattered more than "how thick are your walls?" Walls bought time. Time was what you spent waiting for the other side's clock to run out first.