Catapults were perhaps the best-known medieval siege engine, used to fire projectiles. They were used both to attack and defend castles.
A catapult was usually made of wood. Its main components were a base (sometimes wheeled, sometimes stationary), an arm (held under tension), and a bucket. To operate it, a soldier placed a projectile in the bucket, pulled back the arm with the use of a winch and rope, and released the tension. This caused the arm to move forward with great force, propelling the projectile large distances through the air.
Projectiles were usually made of wood or stone. Occasionally they would use metal balls (for greater damage), dead bodies (to spread disease), or possibly even the heads of dead enemies (for intimidation).
Unlike the trebuchet, the catapult was tension-operated.