Skywatch Castle is a new castle just west of Shor's Stone.  It is quite secluded in the mountains as to not ruin the environment with a huge fortress and I have used the Palace of the Kings Windhelm castle style instead of the overused fort Dawnguard look.  It currently consists of a library, main hallway and a tower interior.  A book with more backstory information can be found within Riften along with a key to the Castle.  It is in major development and the mod doesn't add much to the game at the moment.