Can you create engines

I want to make a tank/onager thing. it's going to metal with a barrier around it with that shoots big boulders and very accurate

Here's the problem you have to figure out:
- The crew will jump out to fight if the enemy got to melee range
- You cannot mod the firing rate
- You cannot mod the speed the onager is moving
- It's no easy matter to mod the siege crew's animation routines so they don't end up cranking your tank from outside before firing, but it's possible
- The siege engine cannot run over anything

I'd suggest using a chariot setup to do this, not an engine.