Medieval designs include a large number of catapults such as the mangonel, onager, the ballista, the traction trebuchet (first designed in China in the 3rd century BC and brought over to Europe in the 4th century AD), and the counterweight trebuchet (first described by Mardi bin Ali al-Tarsusi in the 12th century, though of unknown origin). These machines used mechanical energy to fling large projectiles to batter down stone walls. Also used were the battering ram and the siege tower, a wooden tower on wheels that allowed attackers to climb up and over castle walls, while protected somewhat from enemy arrows.