I like it but there are a few problems i found. for one, THE WHEELS DONT TURN!
I tried to fix them but i couldn't find out what was wrong. Another thing is you dont need the sliding bar on the top of the frame. That is what the wheels are for. Im pretty experienced with trebuchets because since i was about 5 my dad had an awesome F.A.T. that threw over 1,000 feet. And i have made a few of them. Here's an Example:
http://www.youtube.com/watch?v=2D_ifv-sQAs