The Roman army was large and powerful, but it didn't end up that way overnight. They worked hard and, since the Romans are known for their excellent engineering skills, they tried to stay one step ahead of the enemy. One way to do this was with siege weapons, which was how they took over castles and lands. Siege engines ingeniously used both potential energy, kinetic energy, and rotational kinetic energy to throw things very far, quickly, and even accurately. The besiegers could throw 100-200 pound stones up to 1,000 feet. The main siege weapons were catapults. The catapult was used to destroy buildings and walls inside and outside the castle, and could also destroy enemy morale by throwing severed heads of comrades. The first model was the trebuchet. It started by using a large weight on one end of a swivel arm. The arm was pulled back, the missile was positioned and then let go. The weight dropped, the arm went, and the missile was launched. The later model drew its power from a tightly wound skein of rope, hair, and skin. the skeins were twisted incredibly tightly and then had a wooden arm up to sixty feet long placed between them. The arm was pulled back using pulleys and ropes, the missile was placed in the wooden cup, and then the arm was released. The arm jumped at a 90 degree angle where it was stopped by a large piece of padded wood. The weapon was then lowered again and fired again. Onagers were used by the Roman army, every centurion had them. As their rocks were hurled, entire sections of stone walls collapsed. They were great for sieges. The ballista was like a much larger version of the crossbow used by besiegers. His power was fired with tendon cords and he had two arms. Although the ballista was a great weapon, it was difficult to build, so the Romans developed a new version called the Mangonel. He too, like the Ballista, draws his power from the tendon cords but unlike the ballista which used two arms, the Mangonel only used one arm. To make the Mangonel easier to transport, the Romans added wheels. It was very light and could throw objects such as rocks and even fiery projectiles. Another type of catapult is the Onager. Onagers were used until the Middle Ages with the trebuchet, when gunpowder and cannon were invented and eventually replaced catapults.