Beast's Castle is a playable world in Kingdom Hearts: 358/2 Days and Kingdom Hearts II. While this is not a playable world in Kingdom Hearts, you meet up with Beast and help him save Belle in Hollow Bastion.
 Kingdom Hearts: 358/2 Days
Roxas visits this world many times throughout the game to complete missions. Many of these mission are with Xaldin (explaining why he appears here in Kingdom Hearts II). Many of Xaldin's comments, in fact, foreshadow the events that will happen here.
 Kingdom Hearts II
Sora, Donald and Goofy arrive and hear a fierce roar. They instantly remember Beast. As they enter, they go into a room with a glowing rose. Heartless suddenly attack, and begin to overwhelm Sora. Beast arrives and crushes the remaining Heartless and shoves Sora aside to get his rose. They try to follow Best to his chamber, but the corridors are blocked. They go into the dungeon and free Beast's cursed servants, who are all in the form of some household object. They thank Sora and ask him to help Beast by showing him through a secret passage to his chamber. When Sora arrives, Beast attacks him on the spot. Xaldin, an Organization XIII member, has Beast under some kind of trance of anger, and Beast cannot control himself. Sora fights him and ends his blind rage. Beast suddenly feels remorseful for all that he has done. They go to Belle's chamber and let her out (she was locked in there). The Heartless attack again, this time with a large boss Heartless. Sora defeats it and the world is safe for the time being.
On the second trip, Xaldin is there again. He sends Dragoons to attack in the midst of the moment. Short-tempered, Beast is angry and blames Belle. Belle is hurt and she leaves to her room and is kidnapped by Xaldin. He also steals Beast's enchanted rose. Beast goes to his chamber feeling hopeless about the situation, but Sora raises his spirits and convinces him to pursue Xaldin. They go out to the front of the castle grounds, and Xaldin gives Beast the choice of either having Belle or the rose. Fortunately, Belle manages to escape from his clutches and takes the rose back as well. Sora, Donald, Goofy, and Beast fight Xaldin and vanquish him once and for all.
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