The ancient people, long time ago, out of boredom during their passive sieges, invented the first torsion catapult. The rest was history... The invention of the catapult can be attibuted to the time of Dionysius I and the aftermath of the Peloponnesian War, around 400 BC, during the struggle between the Greece and Carthage over Sicily. These actually looked quite different from what we today call a catapult, as they likely shot arrows and looked more like a crossbow, but they did use torsion, and was the first time such a weapon was employed.
These were awsome weapons, which made sieges all over the world more exciting. Siege engines were capable of hurling large stones weighting up to 100kg for distances of several hundred meters. Some shot smaller stones or large bolts (more like spears), which would be devastating when hitting people, while the large ones were used to reduce walls of fortified cities and later castles. There are a number of different engines that were employed, such as the catapult, trebuchet and a ballista. Also used of course were siege towers (which often had a few catapults on them) and battering rams to break down the walls or gates. Undermining would come in handy to collapse a wall curtain or a tower. Of course, the besieged were free to use these weapons agains the besigers too ! Once in a while severed heads of the enemy would serve as amunition...
I will try to build a small model of each one of these engines over time. The first one, a catapult, can be seen below. It is only 9" long, but can lunch small objects between 20-25 feet. My (unsevered) head can be seen on the first picture to get an idea of the size. The second one, a ballista, is a bit larger. Its is about 17" high and 21" long and took me bit more time to put together. This one shoots spears instead of stones.
Torsion Ballista