For the Roman catapult, you want what used to be called an "onager" (which meant 'mule' b/c it kicked when fired like one)

http://members.lycos.nl/onager/onager.html

http://www.howtobuildcatapults.com/catapultonageranimation.html

http://www.roman-empire.net/army/onager.html

And for building a trebuchet....

Testing engineering differences... http://www.globalspec.com/trebuchet/

http://www.tbullock.com/trebuchet.html

http://www.io.com/~beckerdo/other/trebuchet.html