From the Super Mario Wiki
Switchback Hill, or Meringue Clouds-3, is the third level of Meringue Clouds in New Super Mario Bros. U. It is unlocked via completion of Land of Flying Blocks and its own completion opens the path to Slide Lift Tower.
The level takes place in a sky of white clouds and grassy platforms. The level is also infested with Bullet Bills. The main gimmick of this level are Switchbacks that help the player progress through the level.
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