Walkthrough:Kingdom Hearts II/Disney Castle
The first time you'll see Disney Castle is in a small cutscene after completing all three of the previous Disney Worlds (The Land of Dragons, Beast's Castle and Olympus Coliseum).
Upon landing in the Gummi Hangar, Donald and Goofy will become concerned about their world, saying that it's too quiet in there. Chip and Dale will suddenly appear and will tell the party that Queen Minnie needs to see them. There is a save point here, so save your game. Upon entering the Courtyard, you'll find mutiple Shadows, Soldiers and Minute Bombs all over the place. There are also some Treasure Chests in the area, so make sure to open them before you leave the area. To reach the door into the castle, go left and follow the path until you reach a corner with another path. That one will take you to the door. Inside the Colonnade, a seemingly-endless amount of Shadows will attack the party; using Fire or Trinity Limit can quickly dispatch them.
Soon, you'll reach a large door with a seal. Don't worry about it, as you need to find Queen Minnie first. She can be found in the Library, which is at the end of the hallway. Upon entering the room, you'll encounter a Moogle Shop, a save point, and a treasure chest containing a Torn Page. If you would like to return to Hollow Bastion to restore the page, you can do it now, but it's completely optional and you can leave it for later.
Click here to do a quick visit to 100 Acre Wood >>>>
Talk to Queen Minnie. During the cutscene, she'll explain that she wants the party to see something, and asks for them to take her to the Audience Chamber. They agree, but she'll then realize that she forgot to warn everyone else in the castle about the danger. Donald and Goofy then volunteer themselves to warn everyone while Sora (who didn't go because he'll just get lost) stays with the Queen. At this point, Donald and Goofy will leave the party.
Now, you must escort Queen Minnie towards the sealed giant door from earlier while protecting her from the Heartless. The Heartless that will attack include Shadows, Minute Bombs and Bolt Towers. Minnie will release some light spells to fight the Heartless. She and Sora will also have an attack they can perform together called "Faith" using the reaction command button. When you are a few meters away from Minnie, use "Call Over" with 
(in which Sora says "This way") to call her over, where she will follow you.
After reaching the door, Minnie will use her magic to unlock the seal. A small door will then open. Just go through it. In the Audience Chamber, a whole bunch of Bolt Towers will float down. Now you must escort Minnie to the throne. Again, use "Faith" to use Minnie's special attack, and "Call Over" to get her to move. The number of Bolt Towers in this area aren't unlimited so you can choose to defeat all of them to gain some experience. Upon reaching the throne, Sora will obtain another Accessory Slot, along with Auto Summon ablity. In the cutscene, Queen Minnie will open a hidden room, which will release a blinding light that will destroy all of the remaining Bolt Towers. They go downstairs.
This room is called The Hall of the Cornerstone, which now, sadly, had thorns all over it. It's revealed that Maleficent has somehow weaken the castle's defenses and plans to take it for herself. Since this has never happened before, the Queen has no idea on what to do. Goofy then sugests that they consult Merlin the Wizard about the problem. However, you will need to head over to Hollow Bastion. Use the save point in this room to enter the Gummi Ship and head for Hollow Bastion. Upon landing, a cutscene with Merlin will begin, and you will automatically be taken back to Disney Castle.
You'll soon learn that the thorns are the result of someone's medding in a special world. Merlin will bring up a door that will take the party there. Your main objective in this world is to find a door that resembles this one and seal it. Once you regain control of the party, select "Open the Door" reaction command, which will then show a brief cutscene of Sora opening it, then walking through it...
