Manning the cannons on top of a fortress?

I line up some troopers as close as I can to the cannons on the walkway of a fortress but how do command them to use the cannons? Sometimes they do - and sometimes they dont.

you can't manually control it

all you can do is put your men on the wall and they'll automaticly assign to a closest cannon.

in fact, if you right right click on an enemy (attack) they'll change their position and manned the cannon that was pointing toward the target and start firing.

still, i wish the siege battle could work like S2TW tho