I want the patrolling troops to engage outside provinces attacking troops first. If they loose then let them engage the storming troops. If troops inside the castle join the effort by outside province forces then let all troops engage.
I put storming troops in a position that I would not for defending a province. This causes the loss of archers. Also it can mean my front line storming troops are no there when the castle battle comes.
So I want a dual battle system for sieges where a patrolling force is in position to intercept attacking armies.