This bit always bothered me. "We must spend all out defenses on the one pass into Kanyou! The only way to get to the capital!
Oh besides this other way in. That the enemy didn't bother to split up and attack from the start.
Btw, Shin and co. have been making regular trips to the capital, right? So why did they act so surprised and impressed by Kankoku pass before? Were they always using the side entrance?