Unlike ancient artillery, which relied on torsion (twisting) to supply ballistic force, medieval trebuchets utilised a simpler lever action to propel projectiles. Like a seesaw, once one side of the lever was forcefully brought to the ground, by pulling on ropes (traction trebuchets) or a weight (counterweight trebuchets), a projectile – most commonly a stone – was released from a sling attached to the other side.
The traction trebuchet probably originated in China between the fifth and third centuries BC. When it was introduced to the west, and by whom, is unknown. Trebuchets may have been used by Germanic tribes attacking the Roman empire, but more reliable evidence comes from an eyewitness to the Avaro-Slavic siege of Thessaloniki in 597 and from Islamic sources of the late seventh century.
The counterweight trebuchet, also a Chinese invention, replaced the traction trebuchet in the middle of the 12th century and remained in use through the 15th century. It was able to hurl a projectile up to 300 metres but the steep arc of its flight limited effectiveness against fortifications. This has led to the idea that trebuchets were used mainly to launch items over the walls, including diseased animal carcasses or the heads of captured men.
Answered by Kelly DeVries, professor of history at Loyola University Maryland.