Manuballista

[Flavivs Aetivs]

There was one found at Xanten. They were rather uncommon, being artillery, but here are some pics:

http://www.romanarmytalk.com/17-roman-mi...tml#321754

The Late Roman Arcuballista on the other hand was more common, but that was 3rd century AD onwards and would become the medieval crossbow.

I'm sure Dr. Campbell can elaborate.