Your Mission: Clear out the Bandits and you can keep the Castle.
The Cavern is accessible from a boat in the Lake Ilinalta near Falkreath (near Lakeview Manor in Hearthfire DLC). No DLC are needed. Once inside you must vanquish the Bandits who have settled in the Fortress. Each Bandit has a key to the Castle. The Castle comes with a Servant and 2 Followers, Main room, Kitchen and Pantry, Armory Cellar, and Bedchamber. All utilities are included. The  Bandits do not respawn.
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