...Or simply transfer assault tower operations to another gate/wall area away from your strating position, then land a holding force to capture the gate and buy time while you pile your army into the gate. Once you move your entire army into the city, move down your wall force if they have not engaged the wall defenders, and leave them to die if they have. Just mass-charge to break the enemy within the city (since they're romans they have no phalanxes so use massed cavalry to break them and follow up with infantry) and bulldoze your way to the town square, and hold for 3 minutes. No finesse, no elegance, but with no quality and no archers you can't hope to demonstrate the art of assault.

I must disagree with HarunTaiwan's assertion that you divide your forces. It's the no.1 military rule that you should never divide your forces, especially in this case when the enemy is qualitatively stronger than you and will take your split forces apart even when he has split his. One on one hastati > iberian infantry/town militia. Your best bet (and the one with minimum casualties) is to form your troops into a massive battering ram and wear down the enemy via cavalry charge and demoralisation.