Of all the Medieval siege engines, the Trebuchet was by far the most accurate and complex. Trebuchets were capable of throwing 350 pound objects over castle walls. While in this Instructable we won't be making a Trebuchet with that much firing power, the design I will show you is capable of throwing a softball 50-60 feet with a 20 lb counterweight.
Video of me shooting the catapult:
(Sorry the video is sideways, I was having technical difficulties)