Machines for different jobs
No single machine could take a fortress. Medieval engineers used a small set of devices, each suited to a particular problem: smashing a wall, throwing missiles over it, breaking a gate, or getting men onto the wall top. Records are patchy and medieval writers often used terms loosely, so what follows describes the general picture rather than exact designs for every place and century.
Torsion and tension throwers
Late Roman armies used torsion engines, which stored energy in twisted ropes or sinew. Some of that technology survived into the early Middle Ages, but it was complicated to maintain. The word “mangonel” was later used for a range of throwing machines, and historians debate which exact device a chronicle means in each case. In popular books it often means a simple arm-and-sling thrower powered by twisted cords; in medieval sources it can also refer to other stone-throwers.
These smaller machines could hurl stones of modest weight against defenders on the wall, and their rate of fire was better than that of larger machines. Their usefulness against thick masonry was limited.
The traction trebuchet
The first trebuchets were powered by people. A long beam pivoted on a frame, with a sling on the long end and a bundle of ropes on the short end. A team pulled down on the ropes, and the sling whipped a stone forward. Machines of this type are associated with China and then with the Middle East and Byzantine lands, and from around the 6th century they appear in the wider Mediterranean world. The throw depended on how many people pulled and how well they were drilled, which limited the weight of the missile.
The counterweight trebuchet
The counterweight trebuchet replaced the haulers with a heavy box of earth, stone or similar material fixed to the short arm. It is generally dated to the 12th century in the Mediterranean region, though precise origins are disputed. A winch raised the weight, the machine was loaded and released, and the falling weight swung the arm and sling.
Counterweight trebuchets could throw much heavier stones than traction ones and with steadier results, because the force was the same every shot. Reports describe stones of many dozens of pounds, though the size claims in chronicles should be treated carefully. Edward I’s large engine at Stirling in 1304, nicknamed the Warwolf in the sources, is a well-known example of a machine built on site by specialist carpenters. Such engines needed timber, iron fittings and skilled crews, so a big siege often began with building them near the target rather than carrying them.
Besides walls, trebuchets threw other things. Rotting carcasses and other waste were sometimes launched into besieged towns to spread fear and disease, and heads of captured prisoners are recorded in some accounts. How much real effect these had is uncertain, and the more extreme stories deserve some skepticism.
What stone-throwers could and could not do
Heavy stones, fired repeatedly at the same spot, could crack a wall or topple a parapet. But a well-built wall of thick masonry might take weeks of battering to open, and defenders repaired damage at night using timber and rubble. Engineers therefore often aimed at the weakest points: a gate tower, the top of the wall where defenders stood, or a corner where the masonry was less tied together. These weak points were part of what shaped castle architecture, with thicker walls, rounded towers and sloped bases appearing partly in response.
The battering ram
A ram was a large timber, sometimes tipped with iron, swung or pushed against a gate or a weakened section of wall. Operators needed protection from stones, fire and boiling liquids, so the ram was covered by a wooden shed with a roof of hides or wet material, sometimes called a “cat” or “sow” in medieval sources. Rams were most useful against wooden gates and thinner walls, and they were of little help against a thick stone tower.
Defenders tried to drop heavy beams or stones on the shed, to snag the ram with chains or hooks, and to burn the roof. This is why gate defenses were multiplied in castles: a portcullis, a narrow approach, and a gatehouse with openings overhead to attack anyone below.
Siege towers
A siege tower, often called a belfry in English sources, was a tall wooden frame on wheels or rollers, covered with hides to resist fire. Its top level was designed to reach or overtop the wall, with a drawbridge or platform that dropped onto the battlements so troops could cross. Archers on upper floors cleared defenders from the wall.
Towers were expensive and slow. The ground had to be leveled and ditches filled, which meant that the attackers worked under fire. A tower in good position could be decisive; one that got stuck in soft ground or caught fire was an expensive failure. Defenders raised their walls or built extra wooden platforms to keep up.
Engineers and craftsmen
Most of the work was done by carpenters, smiths and rope-makers, with a few specialists known as engineers who planned machines and directed crews. The blacksmith’s forge produced fittings and tools for repair, and stonemasons advised on where walls were vulnerable. In large campaigns, kings hired engineers and kept their names on payrolls, which is one of the better sources for the period.
Gunpowder arrives
Gunpowder weapons appear in European sources in the 14th century. Early guns were small and unreliable, and at first they supplemented stone-throwers. As bigger bronze and iron guns were made in the 15th century, they could be more effective than trebuchets against walls and fired on a flatter path. The reasons were partly practical: guns used less labor to operate than a trebuchet, and iron or stone shot could be aimed with more consistent results. The siege of Constantinople in 1453 is often cited as a turning point because of the big guns used there, though historians point out that the city fell for many reasons, and the old engines did not vanish overnight.
Mongol armies used engineering crews very effectively, and the Mongol conquests show how engines and skilled specialists, often recruited from conquered peoples, could be combined with other tactics. The same principles of combining several methods applied in Europe.
Engines in context
The engine was only one tool. Starvation, mines and negotiation usually decided a siege, and engines served to apply pressure, to wear down morale or to open the final path for an assault. The wider story of how they fit together is in the broader account of the medieval siege.