I wonder why they didn't include petards, those explosive devices used to demolish gates and weak points of the wall. I don't think it would've been a balance issue, because random "hoisting by your own petard" as well as fire arrows would've balanced things out. Maybe you could get the use of them by an ancillary from academies or something, so battering rams would still be viable if artillery wasn't available.

I can see a few exploits, though: arming a peasant unit with a petard and sending them to take out some heavy infantry or cavalry even.