The trebuchet was the most powerful siege engine of the pre-gunpowder Middle Ages, using a falling counterweight to hurl stones at a wall with remarkable consistency.
The trebuchet is a siege engine that throws a heavy projectile by dropping a large weight. A beam pivots on an axle high in a timber frame, with a counterweight box on the short arm and a sling on the long one, and releasing the weight whips the sling through a long arc.
It is the most powerful artillery Europe had before gunpowder, and unlike the torsion engines of the ancient world it uses no springs, ropes under tension or animal sinew — only gravity, which does not weaken with weather or age.
Nothing survives. Trebuchets were built of timber at the siege site, from local wood by carpenters, and dismantled or abandoned afterwards, so the entire physical record consists of a few excavated emplacements and counterweight pits.
The frame is a heavy timber A-frame or box carrying an axle several metres above the ground. The beam is unequal: perhaps a fifth of its length on the counterweight side and four-fifths on the throwing side, which multiplies the speed of the tip.
The counterweight is a box of stone, earth or lead, often hinged so it falls vertically and delivers its energy more efficiently. On the largest machines it weighs several tonnes, and it is the single component that determines the engine's power.
The sling is what makes it work. A projectile simply attached to the beam would leave at the beam's tip speed; a sling adds a second lever that roughly doubles it, and a release pin set at a chosen angle decides when the sling opens.
It is a wall-breaking machine. Shot of fifty to a hundred and fifty kilograms is thrown two to three hundred metres, and the method is repetition — hit the same course of masonry until it fails.
Its consistency is what distinguishes it. The same counterweight and the same weight of shot produce very nearly the same trajectory, so once a machine is ranged it can be fired repeatedly into the same spot, which no earlier engine could manage reliably.
It was also used to shoot over walls rather than at them: incendiaries, and — in a number of well-attested cases — the corpses of animals and of the besiegers' own dead, thrown into a town to spread disease and break morale.
Its strengths are power, accuracy and reliability. It uses no perishable tension components, it can be built from local timber by ordinary carpenters, and a well-built machine will shoot all day for weeks.
Its weakness is that it cannot go anywhere. A large trebuchet is built where it will be used and takes days or weeks to erect, so it belongs entirely to siege warfare and has no place at all in a field battle.
It is also slow. A few shots an hour is normal, because the counterweight has to be winched back up between each one, and on the largest machines that is the work of many men or a treadwheel.
The earlier form is the traction trebuchet, pulled by a team hauling on ropes rather than by a counterweight. It reaches the Mediterranean world from China through the Islamic lands and is in use in Europe by the twelfth century.
The counterweight machine appears in the later twelfth century, and it is a genuine advance: a weight can be made far heavier and far more consistent than a team of men pulling, so the engine becomes both more powerful and more accurate.
The thirteenth century is its high point, and it remains in use well into the fifteenth alongside early gunpowder artillery. It disappears only when bombards became reliable enough to do the same job faster.
The engine is common to Latin Europe, Byzantium and the Islamic world, and it moved freely between them — crusading warfare in particular spread designs in both directions, and Arabic and Latin sources describe closely comparable machines.
Northern European machines are the best documented in the written record, largely because English and French royal accounts record the carpenters, timber and ironwork paid for at specific sieges.
The vocabulary is unstable and does not map cleanly onto machine types: trebuchet, mangonel, perrier and blide are used loosely and sometimes interchangeably in medieval sources, and modern distinctions between them are largely a scholarly convenience.