From the Super Mario Wiki
Freezing-Rain Tower, or Frosted Glacier-Tower, is the Tower of Frosted Glacier in New Super Mario Bros. U. It is unlocked via completion of Cooligan Fields, and its own completion opens the path to Prickly Goombas! and a cutscene of Peach's Castle being completing shrouded in Kamek's tornado. However, the cutscene does not play if Giant Skewer Tower is completed beforehand.
A fourth Boom Boom is the boss of the tower. Kamek flies in and gives the Boom Boom the ability to jump. Unlike in the second round, however, this Boom Boom keeps his spin attack, even after getting stomped on.
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