Trebuchets were the final stage of development in the line of medieval siege engines; they followed catapults, onagers, ballistas, and such implements, and preceded the changeover to gunpowder. They're also a lot of fun to use to fling things around.
You can build your own Treb for not much money. It did take awhile to put together (many vacation days happily puttering with tools and sawdust), and virtually all the parts came from "normal" hardware stores and building yards.