World 6-Ghost House (New Super Mario Bros. 2)
From the Super Mario Wiki
World 6-Ghost House is the second level of World 6 in New Super Mario Bros. 2.
This Ghost House is a difficult maze, similar to the Ghost House found in World 5 in New Super Mario Bros. Wii. Mario must press the ? Switch at the end of the room and go in the right door. Then, the player must head right until they see a door. Mario should go inside. What looks like the same room is actually different, as the Big Boo has 'turned into' a Boo. Mario must then press the ? Switch and head left. When Mario gets to the first part of the room, he should lure away the Boo from its position, and press the ? Switch that it was hiding. Some stairs will then be visible, and Mario must climb them. At the top is a door that Mario must enter. Then the player will be in a room with the first Star Coin. Mario should not enter the door located on the bottom of the room. He should instead wait and head back up for a block that has a ? Switch on it, on the left side. When it is pressed, a path is made to the exit door.
After the player hits the ? Switch in the room that slides opens the ceiling leading to the third Star Coin, they must wall jump up to the star coin, but instead of heading back down, keep wall jumping for about 3 screens and they will come to a door leading to the secret exit. If the walls are closed after the ? Switch wears off, he will be forced to go to the secret exit.
The Secret Exit unlocks World 6-A.