Here's the secret to get to the secret room!
1. Beat the final level King Bowser's castle
2. Beat a minigame and win a super red watermelon.
3. Go to Secret level 5.
4. Go through the first room.
5. Go down the pipe into the castle.
6. Get past the switch part but when you reach the part where you have to enter a pipe, instead of entering it, jump on top of it then jump on top of the blocks that are on top of it.
7. Hover to the left onto other platforms.
8. When you see a pipe being blocked by an ice block, bust out your super red melon then breathe fire at the ice block to melt it. Enter the pipe. You'll be in a room filled with green melons, bandits and winged ? clouds with 1-ups in it!