The catapult was first designed and used by the Romans around the 2nd Century AD. The prototype of the Medieval catapult was known as the Onager, and the Romans first used it as an anti-personnel weapon. The Onager had a larger and wider firing cradle which housed a load of gravel or stones instead of one rock, projectile or missile. The use of the Onager was to release a hail of stones at a group of approaching enemy infantry to injure and incapacitate as many as possible. The original Onager had an effective firing range of 40 - 80 yards.
It is believed that the Celts were the first to modify the Onager into its more popular form of the catapult. Adapted around the 5th Century with higher torque and stronger and more pliant wood, the Onager was upgraded to the catapult. The Celts were known to use this weapon to breech Roman walls and motte and bailey type castles which eventually led to the Roman retreat from occupied Celtic territory.
During the 11th Century the catapult underwent major changes in engineering and design. The advent of new technologies allowed the weapon to pass from its critically primitive form into one of the premier siege weapons of its time. Mounted on wheels instead of stationary, the catapult became a mobile weapon that could easily be positioned, fired, advanced and retreated. During its height, the catapult could fire a 30 pound stone 200 yards with deadly force and effectiveness. However, the weight and propulsion of the stones made for a lengthy process of siege as a steady and long battery was necessary for results. Thus the catapult remained useful, but other methods of invention were researched to improve upon its idea. Thus the trebuchet was designed and introduced into battle.
The trebuchet was primarily in its developmental stages throughout the 12th Century. Early models were employed without much effectiveness as the devices often broke down and crumbled from the stress inflicted upon the machinery. During the early 13th Century however, the trebuchet was modified and built with increasing improvements. The trebuchet applied the scientific methods of torque and counterbalance. A weighted arm was opposite a netted sling. When released, the weight pulled the sling upward until it hit a mounted crossbeam, thus enabling the created torque to release and fire a stone 400 - 500 yards. The weight of the stone depended on the size of the trebuchet. The devices were built in one of three categories, Light (being the smallest) Medium and Heavy. A Heavy Trebuchet could stand anywhere from one to two stories in height and was by far, the most powerful and effective siege engine of the Medieval Ages.
Truly one of the most deadly and impressive inventions of the Medieval Ages, trebuchets were feared by any who encountered them. King Edward II of England in fact, refused to accept the surrender of his enemies within a castle, just so he could witness the devastating power of his trebuchets at work.
These diagrams offer a closeup view of the counterbalance element employed to fire the weapon as well as the sling in which the rock or projectile was housed. The trebuchet was highly technical and advanced for its day and saw use until the end of the 16th Century.
Click Here To See An Animated Trebuchet At Work