I'don't believe that letting an entire army pass away a fort without much trouble is a mistake.
What's a fort? It's a wooden fortification that never passed the 200 meters long and served primarly to support troops, take care of logistics, armour repairement and few other things except for giving armies a rally point to meet!
Can forts blockade a road so well that an army shall not pass to a province? NO!!! Except the passage is no longer than 400 mts, so archers may have a chance to hit somebody who wants to walk by.

Diego, from Argentina
(still waiting to play RTW!!!!)