The door is useable on the Bullroarer test server but it's not meant to be useable on the live servers, so the devs put a level limit on the door for the live servers that no player can reach. There are more doors like this in other parts of the game, not meant to be useable on the live servers but meant to be useable on the test server. On the test server, there is no level limit and the doors can be used.

As for what's behind the doors, it's the PlayDwarf mansion, bearded lady dwarves in cute caveclaw costumes.