After you get 8 stars you can enter the room on the second floor with a star on it. Enter the room and walk towards the peach picture keep on walking and you will fall through a trap to enter bowsers level. Finish the level by beating bowser and obtaining a key. ONce you are at the entrance of the castle again go through one of the brown doors, go down the few steps then immediatly turn to go down a longer flight of stairs, open the door with your newly obtained key. Walk through the tunnel. and open the door. Turn right and you should see a picture of a flaming head, stand in front of it and turn left, there should be a empty hallway with an empty wall, it's not really empty! Jump through the wall to get to the Shifting Sand Land course!