A few years ago for my 'Technology and Engineering in Antiquity' class, the students were given the option of either writing a mid-term test, or submitting a project about some medieval/ancient device that they had built. Due to my aversion to studying, I decided on the latter.
I ended up building a ballista, which is more-or-less just a gigantic crossbow. It wasn't very effective, but at least it worked! You can read about my project at my Medieval War Weapons blog. It includes pictures and several videos of the ballista in action.