SCA websites would be VERY useful for this sort of thing also. I'm assuming you mean one of the giant boulder-hurling types, usually called either a trebuchet, ballista, or onager. Catapault usually means a smaller machine that projects rocks or arrows.
try:
trebuchet.com,
http://www.tbullock.com/trebuchet.html, http://www.howstuffworks.com/question127.htm