Why not use the corpses/bones of the previous victims?
For large bones and corpses this would be glorious. A single stack of elephant bone falling 1Z level can break a few bones. Of course, falling damage is calculated differently from flying objects. Also launching the skulls of the defeated towards the next invasion just reeks of delicious psychological warfare.
For the smaller ones it would probably be better to carve them into bone arrows and stick those in the minecarts, as it looks like they would act as if fired from a bow.
Currently my main 3-wide trade entrance is covered by 3 minecart cannons filled with galena and misc trap components but I'm wondering if it's possible to have the entrance 2 or 3Z levels high, so I can have the minecarts fire over a nice little array of ballistae-- but that looks as if all the shot would scatter
over the heads of the enemy, instead of
in the heads of the enemy.
Looks like further testing and experimentation is required.
First test fire of the standard configuration:
Woodcutter ignored path restriction during testing.
Lost one arm and beheaded by a flying -iron serrated disc-. He's smeared across the entrance as a reminder to others-- and since it's so effective it seems that I don't need a militia captain anyway.
Also it seems that having the depot too close is a bad idea. Shotgunned the caravan and my broker.