As i said (MANY MANY TIMES BEFORE) for some siege engines i've take inspiration from brickshelf and that's the case of the ballista.
From the homepage:
"Marco, better known in our forum as medieval_total_lego, comes from Fano, Italy, a small town on the Adriatic Sea. He's a big fan of medieval history, and has an interest in attacking walled cities (at least in ABS form). Inspired in part by things he's seen on Brickshelf and by units from the game Stronghold 2, he's created his ultimate siege engine collection to outfit an attacking army. Unfortunately he didn't have time to put all of these to work in an entry for the recent Battles Contest, but here we see some of them in one of his CCC entries. Hopefully soon we'll see a larger scene incorporating all of these elements.
In addition to CC, Marco is a member of the Italian group ItLUG. His LEGO creations can be found on his MOCpage and Brickshelf sites. "