I only leave dragoons to keep order, and move my dedicated armies around to deal with incursions.

In one campaign i wanted to play a little with defending in sieges, so I placed a small garrison in the first level of city fortification in a border province. I think I had a few units of line, 2 mortars and 2 howitzers. Never got to defend too many times but that setup can basically annihilate the AI. I put the freebie units on the walls to man the cannon there, and keep my line units facing the walls in the center. If the enemy get onto the walls I lose nothing and they get shot while making their way down to the center. If they breach the walls, well you know the story I guess. Just move the infantry to box them in at the breach and shoot them while in melee and you get a massive rout. All the while they're being blown up by explosive shells.