How to get mushroom world
Go to world 1 mini-castle. A little bit after the checkpoint, go into a door with a cross on it. Instead of going into the other door with a cross on it, in the other room, go into the hole in the middle of the wall on the right. There should be a flag somewhere near. Once you land on the flag and have completed it, a path to a cannon should appear. Do the cannon and you should be in world mushroom.
More tricks, tips and cheats for this game are right here - New Super Mario Bros 2 cheats
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