EDIT: the ballista adds more to the RP fun they will be further away from seyda neen and must be destroyed or else they will just take us all down with many well aimed bolts.
I think they should be made of Wood and metal, a solid metal Ballista would way tons and be extremely hard to move. One with a wooden from and metal mechanical parts would be much lighter and more practical. They could possibly even have automatic steam operated reloading systems?