In World 8-Castle of Koopa, go in the highest door at the end of the Donut Lift room. You should be in a room with more Donut Lifts, Bowser statues, and fireballs shooting at you at the end. Squat and fly (easiest with a P-Wing) and land on top of the last Bowser statue's head. Stand up and Mario will pulled through the wall to another room! It's not really a new room; it's the room you can find by taking the lower route after all the Donut Lifts.