Within the Castle Walls
After you get all 120 stars, the cannon on castle grounds will be open. Load yourself in and launch yourself onto the castle's roof. Hit the red ! box there and put on the Wing Cap. Fly down to castle grounds and get inside the cannon. Fly up to where the roof meets the tower, and walk up to the wall. You'll fall, yet you'll lose half off of your health meter or more. Walk in the door, and you'll see the inside of the castle from in the walls! Say hi to the princess for me!