It does seem to me that fire-ballistas light up rams and towers more often than anything else. Cannons seem to be almost useless against rams, in my experience, but they work wonders on towers and the enemy army. When it comes down to it, you simply need to remember that, unlike in RTW, you cannot count on destroying all their siege equipment. You can get some, but you had better prepare to defend your walls and a breached gate, otherwise you're in trouble. If you defend your gate area and then manage to burn the rams, all the better... move your gate defenders to the walls.

If you can't defend the walls and the gate at the same time, I suggest you do what damage you can and then retreat to the streets or the town square.