Hidden Bowser Castle
From the Super Mario Wiki, the Mario encyclopedia
After recovering 5 of the 6 Royal Stickers, Mario must travel to Gate Cliff. Here, the Royal Stickers will begin to react, illuminating stronger than usual. The player must then touch each of the Royal Stickers. Doing so will reveal 5 points on the cliff Using Paperization, Mario must peel off the pentagonal shape created by the Royal Stickers. The scrap will flip around, revealing an image of Bowser. Mario must then paperize again and place the scrap back onto the cliff, which will reveal Bowser's Sky Castle.