While lurking around the net I stumbled upon this interesting page about catapults (ballistas)
It's a bit outdated (2008), but it proved an interesting reading; it both depicts the use and the costruction of a roman catapult (an variants); I'm not very deep inside the matter, but I think it can be a good source for ye historians; also shows quite a good amount of close up pictures, useful for our belowed modders I guess