(Sources: Storm King’s Thunder, page 111, Sword Coast Adventurer's Guide)

In the cannon depiction, it is rumored that there is a cave under this unadorned castle that leads into the Underdark, which is why the Dwarves that run this castle have restricted areas from any visitors. In my version, it's no rumor: The Dwarves there are protecting the surface region from a very real path to the Underdark--where there is still a problem with demons ousted from the Abyss--which they also use as a mine and a workshop. They are still very cordial to the public and keep up a hospitable waystation for travelers along the High Road.

Micro-Worldbuilding Opportunity: That mine that leads down to the Underdark.