After the demonic invasion, the few survivors live here, as the castle is the only place in wich the demons weren't able to breech.
Apart from the queen, her general Sir Krum and two patrolling guards, the castle homes various, like Damian, who sells miscellaneous weapons, potions and bombs, Irene, selling rare (yellow) weapons, Patriarch Casimir, seller of Destiny Cards and Private Stash, who offers to deposit and retrieve items.
Outside of the castle's inside, there are the Royal Gardens, overrun by demonic spiders and undead, with boss monsters like Argus the Dread Mage and the Chessmaster.
The Royal Gardens homes various crypts, like the Befouled Tomb, the Sepulcher of the Damned, the Royal Crypt and, later on, the Den of the Broodmother.
Near end game, the castle will be attacked by the Wraith boss Apocalypse.