A mangonel (derived from Greco-Latin word manganon, meaning "engine of war") was a type of catapult or siege machine used in the medieval period to throw projectiles at a castle's walls. The exact meaning of the term is debatable, and several possibilities have been suggested. It may have been a name for counterweight artillery (trebuchets), possibly either a fixed-counterweight type, or one with a particular type of frame. The Arabic term manajaniq comes from the same word, and applies to various kinds of trebuchet. It is also possible that it referred to more than one kind of engine, in different times or places, or was a general term.
In modern parlance mangonel is often used as the name of a Medieval form of onager, though there is little evidence for this historically. In this sense, mangonel had poorer accuracy than a trebuchet (which was introduced later, shortly before the discovery and widespread usage of gunpowder). The mangonel threw projectiles on a lower trajectory and at a higher velocity than the trebuchet with the intention of destroying walls, rather than hurling projectiles over them. It was more suited to field battles.
Mangonels shot heavy projectiles from a bowl-shaped bucket at the end of the arm. The bucket could launch more rocks than a sling; this made it different from an onager. The sling was changed to a bucket for more force when firing. In combat, mangonels hurled rocks, burning objects (fire pots, vessels filled with flammable materials that created a fireball on impact), or anything else readily available to the attacking and defending forces. The more unusual types of projectile included dead, and often partially decomposed, carcasses of animals or people, used to intimidate, demoralize, and spread disease among the besieged. This tactic often proved effective. (From wikipedia)