Suggestion for choke points on campaign map

Have an on/off option for certain castles (like a choke point castle) and their area of influence.

When turned on, that castle's area of influence cannot be crossed unless you're in possession of the castle or allied with the faction that possesses it. This would be a great feature for castles blocking a mountain pass for example, where it wouldn't make sense for an army to just go through it on the campaign map since there is no feasible way to go around it.

When turned off, armies could go through that castle's area of influence, but the army could be exposed to attack from the fort perhaps, slowed down, famished, etc.