The Roman army was large and powerful, but it didn't get that way overnight. They worked hard and, as the Romans are known for their excellent engineering skills, they tried to stay one step ahead of the enemy. One way they did this was to use siege weapons, which allowed them to take over castles and lands. Siege engines ingeniously used both potential energy, kinetic energy, and rotational kinetic energy to throw objects very far, quickly, and even accurately. The besiegers could fire stones weighing 100 to 200 pounds 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 it could also destroy an enemy's morale by throwing the severed heads of their comrades. The first model was the trebuchet. It all started by using a large weight on one end of a swing arm. The arm was removed, the missile was placed and then released. The weight came down, the arm went away and the missile was launched. The later model drew its power from a tightly coiled skein of rope, hair, and skin. the skeins were incredibly tight 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 shot out at a 90 degree angle where it was stopped by a large padded piece of wood. The arm was then lowered and pulled again. Onagers were used by the Roman army, every centurion had one. When their stones were thrown, entire sections of stone walls collapsed. They were perfect for seating. The ballista looked like a much larger version of the crossbow used by the besiegers. It drew its power from taut ropes and had two arms. Although the ballista was a formidable weapon, it was difficult to construct, so the Romans developed a new version called the Mangonel. Like the Ballista, it also drew its power from tight ropes, 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. She was very light and could throw objects such as stones and even flaming projectiles. Another type of catapult is the Onager. Onagers were used until the Middle Ages with the Trebuchet, when gunpowder and the Canon were invented and eventually replaced catapults..