I don't know what the problem is, because my guys man the cannons and the ones who arent on the cannons are musket-firing the living dog crap out of the people coming to and up the wall. Even if they arent facing the wall or evenly distributed they still do this, although the longer the line the more muskets that fire.

Deploying in the star segment is tricky there are only a couple small spots you can do it and it must be a small, squarish formation. Otherwise you have to run them in.

When the attackers breach the top of the wall all nearby units will leave the guns and move in to fight if you change to melee. MAke sure fire at will is back on before you put them back on cannons.

I'm actually having the exact opposite problem as you.... they aren't coming up my walls, they are running their horse artillery up close and battering down a door or wall and coming through in a giant horde of melee while i still have people defending on the walls. My last few defenses i've actually run my horses outside to get away and made them take the walls from the inside out, which makes for some pretty intense stairwell fights