Trebuchets are cool, but I prefer the big ass cannons. They also have catapults, centrifuge-like machines, and human-powered stuff.
A trebuchets I think drops a weight and has the thing to be thrown on a whip that uses centrifugal force to throw the object. I think these go back to either roman or greek siege machines. Big risk with a Trebuchet is when the load fires backwards instead of forwards.