What would be nice as well is to have the fort being useful. At the moment you can build a fort and the enemy can passe just in front without your men moving or even worst, if you are under attack you start inside the fort. The garnison should have the choice of "intercepting" the passing army or the fort should block a surface around him.
I build a fort near the pyrenee (north of spain) to block any incoming army from spain. I stack like 400 men in it, and let it. Then I realise it was useless when the closest city from the border was under siege. A spanish army of 300 men just pass without me noticing it....