How Do You Break Into a Castle? Machines Built on the Spot

A fortification converts a military problem into an engineering one, since walls too high to climb and too thick to break force an attacker either to go around, to starve the defenders out or to build something. Starvation was the commonest method and the slowest, exposing the besieging army to disease, desertion and relief forces, so the pressure to break in quickly was considerable. The machines that developed address the wall in three ways, by going over it with towers and ladders, through it with rams and projectiles, or under it by mining. Each approach had counters, and the history of fortification is a sequence of responses, with walls thickened, ditches dug, towers projected to permit flanking fire and approaches cleared.

The devices are fewer than popular depiction suggests and each does one job:

• •The battering ram, a suspended timber swung against a gate or wall, protected by a roofed frame
• •The siege tower, a mobile wooden structure rolled against the wall to deliver attackers at its top
• •The torsion catapult, storing energy in twisted rope, inherited from classical practice
• •The traction trebuchet, powered by a team hauling on ropes, arriving from the east
• •The counterweight trebuchet, using a falling weight, which was the most powerful pre-gunpowder machine
• •The mine, a tunnel dug under a wall and collapsed to bring it down

Transporting large machines was impractical, so armies carried the specialists and the metal fittings and built the rest from whatever timber was available near the target. That fact explains a great deal. It made the presence of woodland near a fortification a strategic consideration, and defenders cleared ground around their walls partly for that reason as well as for visibility. It meant sieges began with a period of construction lasting weeks, during which the defenders could act. It made carpenters and engineers among the most valuable people in an army, frequently recruited specifically and paid accordingly. And it meant the machines were left behind or burned afterwards, which is why almost nothing survives archaeologically and why reconstruction depends on manuscript illustrations, written descriptions and modern experiment.

Fortification developed as a direct answer to each of these methods and the responses are legible in surviving structures. Walls were thickened at the base and sloped outward there to deflect projectiles and to make undermining harder. Ditches, dry or flooded, kept towers and rams at a distance and complicated tunnelling. Towers projecting from the wall line allowed defenders to shoot along the face of the wall at anyone working at its base, which a straight wall cannot do. Concentric arrangements put a second wall behind the first so that breaching one achieved nothing. Countermines were dug to intercept attacking tunnels, occasionally leading to fighting underground. Defenders also built their own machines on the walls, which had the advantage of height. Gunpowder eventually overturned all of it, replacing high walls with low thick ones set behind broad earthworks.

The most effective pre-gunpowder machine deserves a description because its principle is simple and its performance was not. A long beam pivots on a high frame with a heavy box on the short arm and a sling on the long one, and releasing the weight swings the beam, accelerating the sling and releasing the projectile at the top of the arc. The sling roughly doubles the effective length of the throwing arm, which is where much of the range comes from, and the release timing is set by the geometry of a hook. Full-scale reconstructions built since the 1980s throw stones of a hundred kilograms and more over more than a hundred and fifty metres, with accuracy sufficient to strike the same section of wall repeatedly, which is what actually brought walls down. Gunpowder artillery displaced these machines over the fifteenth century.

Walls force an attacker to go over, through or under, and the machines address each of those in turn. They were built at the siege from local timber, which is why sieges opened with weeks of construction, why cleared ground mattered and why almost nothing survives. Full-scale counterweight reconstructions throw hundred-kilogram stones over a hundred and fifty metres accurately enough to repeat a hit.