Roman Ballistae in Modern Popular Culture

[P. Clodius Secundus]

Even mounted on a cart (especially ox-drawn), they would not have the same degree of maneouverability as archers on horseback, or even on foot. Given their range and power that's probably not so much of an issue though.

As far as I know, the only evidence points exclusively to their use with mule carts, not oxen. Of course that doesn't mean it never happened, just that it wasn't established tactical doctrine. Relating this back to the original theme of popular culture and it's misconceptions, one of the most common myths is that the mule drivers or "gunners" rode in the carts with the weapon. Most miniatures companies perpetuate these errors.
Vegetius describes the carroballistae as being deployed well to the rear and flanks of the infantry line, and firing over their heads. This put them out of range of slingers and archers (keep in mind, we're not talking about English longbowmen). Like the advent of galloper carriages or light horse artilley the new iron-framed ballista seems to have moved artiliery from the siege trains, often ox powered, up into the forward elements. This is a much better fit with the Roman's offensive style of warfare. To me, the coolest thing about the images on Trajan's column is that they show the weapons being deployed in three distinctly different ways; Static within the fortifications as "mural" or wall artillery, mounted and firing while still in the bed of the cart, and forward deployed beyond the main camp atop wooden cribbing. It would take gunpowder artillery many years to develop a weapons system this versatile.