Sapping should be darn easy to add since it's already in the Rome engine. I honestly think that sieges become a lot easier due to the gates becoming very viable. Whereas in RTW best way was to fight on walls with siege towers to avoid boiling oil of doom, now it's walk up, ram the gate, and then zerg your way in.

Sieges seem biased against humans. With equivalent defenses, they set fire to my equipment far more than I do to them. If I assault using a single ram against even wooden walls, it's practically guaranteed to be burned. Meanwhile, I need at least ballista towers to be able to be fairly certain that their ram dies.