RTF is a global modification that replaces all towns in the game.

how about making the ballista magical, placing an adept and a dwarf, and the adept simply teleports the entire structure a few squares. Not across the entire field, as that would be too powerful, but say, 4 squares. This could be explained in the lore by saying that the ballista is huge and difficult for a mage of that level to teleport, while a hero could move it as and where he pleases using a spell.

Added 2 minutes later
to clarify: I mean making the movement with a teleport animation.