The legion also carried an artillery detachment with 30 pieces of artillery. This consisted of 10 stone-throwing onagers and 20 bolt-shooting ballistas; in addition, each of the legion's centuries had its own scorpio bolt thrower (60 total), together with supporting wagons to carry ammunition and spare parts. Bolts were used for targeted fire on human opponents, while stones were used against fortifications or as an area saturation weapon. The catapults were powered by rope and sinew, tightened by a ratchet and released, powered by the stored torsion energy. Caesar was to mount these in boats on some operations in Britain, striking fear in the heart of the native opponents according to his writings. His placement of siege engines and bolt throwers in the towers and along the wall of his enclosing fortifications at Alesia were critical to turning back the enormous tide of Gauls. These defensive measures, used in concert with the cavalry charge led by Caesar himself, broke the Gauls and won the battle—and therefore the war—for good. Bolt-throwers like the Scorpio were mobile and could be deployed in defence of camps, field entrenchments and even in the open field by no more than two or three men.