I think I've seen the Nova special. I've been a trebuchet fan, but I've been using Onagers in the Rome: Total War game I've been obsessed with, along with Scorpions, Ballistas, and repeating ballistas.
Rome's military supremacy was based on their engineering, I think I'm safe in saying.
That and their troops -- very well-trained, well-conditioned, and extremely battle-hardened. The difference between Roman troops and their enemies is similar to the differences between our troops and those of the salamikazes.