Arrow slits! I thought of that, Ulmish Crossbows could be devastating, but Dom doesn't allow ranged combat in the same turn as trampling, so it would be silly. Perhaps a ballista that you could set to fire first if you knew there were large opponents in the enemy army. Also nice would be large spikes around the base of the Machine, but as far as I know there is also no way to add bonus damage to trampling, so I opted for a sleek look.
Perhaps the spikes could be worked in like some sort of a damage shield? Or we could say they kept a pot of boiling oil inside the cabin of the thing, and could spit it out of aimable tubes on the sides, so just have it behave like a 10dam Fire Shield maybe?