With the barbarian invasions and the fall of Roman Empire, it lost an important heritage of technical skills, essential to military operations in particular for sieges and defenses. It will be the Byzantine and Arab engineering that reintroduce a set of tactics and machines derived from the great tradition of Hellenistic-Roman technology.Ancient jet weapon, the catapult was made up of a beam fixed at one end with an articulated joint, at the base of the machine, while another free beam, where the bullets were loaded, ended in a spoon. The beam was forced down by exploiting the strong cords elasticity, leather twisted strips or bronze springs, accumulating tension. Once released, the arm snapped forward, striking a heavy braking bar: in this way all the bullets on each end of the arm were hurled to the target, following a flat trajectory. The catapult was the only type of machine can be positioned on a tower or a wall, when the trebuchets were too heavy for the walls.