User blog comment:Darkcloud1111/Good Idea, Bad Idea~Mountain Glenn/@comment-5217560-20141013022054/@comment-4010415-20141013032512

I wonder if this all happened before airships became commonplace. It sounds like the only reason the subway tunnels were built was just for the workers to be able to commute from Vale to Mountain Glenn without having to walk through the grimm-infested wilderness. If airships were commonplace at the time, they could've just used those to commute, so maybe they weren't at the time.

By the way, Vale did not seal off the underground city until the grimm invaded the underground city.

The reason they sealed off the underground city was because it was overrun with grimm, and the subway tunnel from there lead directly inside of the kingdom. This means that, if they didn't seal the tunnel, the grimm would've had a direct path inside of the kingdom, which would mean they would have an easy way to invade Vale.