A cannon might be interesting, instead of a siege tower. Have the cannon placed out in front of the wall, immovable most likely. Have several tasks required to use it, such as people have to pick up bags of powder (with an action bar, like ladders), then load it into the cannon (with a longer action bar), perhaps four bags of powder, only one can be loaded at a time. Then someone has to pick up and load ball. Then someone has to pick up a torch nearby and light it. Throughout all this, defending archers and crossbows are trying to interrupt and kill the gun crew, determining how long it will take to fire the cannon. Ladder use is unaffected by this, so the cannon not being fired doesn't stop the assault.
When the shot goes, everyone on the map can hear it, and there is now a big hole in the wall.
That's about as much gunpowder as I'd like to see, and it is more period-correct. Hand-held guns change the game in time and style significantly.