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How about making the ballista magical, placing an adept and a dwarf on it, and having the adept simply teleport 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 wherever they want using a spell.
Added 2 minutes later
To clarify: I mean making the movement with a teleport animation.
Now about the ballista: the iron ballista is out for a simple reason: the only free city with dwarves is engaged in logging, not mining.
The idea of a rapid-firing ballista comes to the forefront, especially since it fires very quickly in the Dark Messiah.
We can change the ammunition for an alternative upgrade – instead of a dart, use a stone (possibly magical)