To get to the back door of Bowser's castle go to Valley of Bowser 2. After you go through the second pipe in the level, u have to run to the right to avoid being smashed by the rising floor. When u get to the end of the floor u should see a megamole. Instead of going through the passage to the right, jump up to the ceiling on the left and run left, u will fall down into a small room with a key and a keyhole. Put the key in the keyhole and clear the level that way. A path will open up to the Valley fortress.
Beat the Valley Fortress and a path will open up to the backdoor.
Prepare yourself for the Great Beyond Mario!!!-Bowser