I suppose you should try silver next then, for the extra blunt force, since they seem to work similarly to crossbow bolts in that density matters more than sharpness. Makes me curious about slade ballista bolts.
Tricky. I don't think there is any silver ore on this site. I'll have to look around and see if I can find any. Alternately, I have modded in a few experimental ultra-dense metals which I might be able to make instead.
Is it possible for vampires to fire a ballista whilst both are underwater?
I'd have to make a completely new fortress to test that, this one was built in 31.25. I suspect the answer is no, buildings which are underwater can't be used even if you mod the dwarf to not have to breathe.
Is it possible to fire a ballista arrows through magma using the technique you used to keep the water in position? !!Ballista arrow!!?
Interesting question. I think I can test that in this fortress, I'll just need to build a lot of magma-safe pumps.
Great work Sphalerite! Is there any reason why the wallgrates couldn't be built further back, nearer to or directly where the pumps are taking the water from? This would prevent them from being completely submerged, and so hopefully confine the creatures.
I suspect that those wouldn't help. Fortifications become useless when completely submerged. With grates it's moving water which makes them not work, any water at a fluctuating level less than 7/7 has a chance to push a creature through the grate. Adding grates where the water is already moving doesn't seem to do much. Maybe a third layer of grates further back in where the water is almost always 7/7 would help instead.