An underground base for magic submarines, around the corner on the far side--there's a bit of a cove there.
Knowledge. It's actually a monastery built in an inaccessible area specifically to deter pesky seekers of truth from showing up and tracking mud all over the floors.Passage. Behind that fortress lies a well trafficked, but narrow road hugging the coastline. It's the easiest way to move armies through the mountains, and this fortress ensures that only the right armies get to do so.