why would this door be here?

[Sven Osymthe]

Before the islands were designed, you could enter/exit the back of one of the taverns. Even though that's not the one pictured.

But yes, as the others have mentioned, they use the exact same models for nearly everything. I guess they don't think people will notice. A while back I went on the boss battle and glitched off, ALL of the forts look exactly like Kingshead (except some bugs that you can't see unless glitched), and the town just contains random buildings found throughout the game.