From the Super Mario Wiki
Crushing-Cogs Tower, or Acorn Plains-Tower, is the tower level of Acorn Plains and the first overall tower in New Super Mario Bros. U. It has large moving stone cogs and Dry Bones. This course is the first to have the Ice Flower. The level is unlocked via completion of Tilted Tunnel and its own completion unlocks Yoshi Hill.
A Boom Boom is the boss of the tower. The boss battle is basic, containing no extra hazards or obstacles.
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