http://tuhsphysics.ttsd.k12.or.us - Site of the TuHS Physics department.  Provides links to curriculum as well as the other research projects completed throughout the years. 

http://www.mangonel.com/ballista/ - Explains the function and mechanics behind the ballista.  Also provides information detailing how you can purchase one.

http://en.wikipedia.org/wiki/Ballista - Gives background information about the Ballista and describes its structure and operating function.

http://www.redrampant.com/ - Provides information on the Roman Empire Army, avid users of the ancient ballista.

http://www.newton.mec.edu/Brown/Te/CATAPULTS/Catapult_history.html - Explains the characteristics of the ancient ballista along with its uses throughout history.