Beauty Castle can be unlocked by passing the bill "Mysterious Seal" in the Dark Assembly. You need a rank 9 character and 5000 mana in order for the bill to be proposed. The Senators of the Dark Assembly dispise this proposal. As such, it is almost impossible to pass this bill without choosing the "persuade by force" option and defeating the opposing senators in battle.
Completing the final map of this area results in recruiting Marjoly as a playable character (not in Etna mode). It is also one of the requirements for unlocking Baal Castle, along with clearing the Alternate Netherworld bonus area.