Castles should have an area of influence

Castles are traditionally placed in strategic locations as opposed to economically important locations. If an enemy wants to, say, go through a pass to reach the other side, they must contend with the castle.

Right now, we can just march around the castle uninhibited. What should happen, I believe, is that you are forced to do battle with the soldiers stationed there (possibly at the discretion of the holder of that castle).

I'm not sure how best this could be implemented, but the easy way is that if you try to pass through the area of influence, you are given the option to go through, but you lose a percentage of troops and are slowed down by a certain amount. The better developed a castle, the larger the area of influence and the greater affect on enemy armies. Perks on the opposing side would enable them to counteract some of these (scouting, tactics, roguery).