From the Super Mario Wiki
Flame-Gear Tower, or Acorn Plains-, is the tower of Acorn Plains, and subsequently, the third course of New Super Luigi U. It is unlocked by completing Crooked Cavern, and its own completion unlocks Rolling Yoshi Hills.
The player begins the level under two Burners. A cog is found, along with a ? Block above a Burner. Another cog with a ? Block above it is seen, along with more Burners. A Hidden Block containing a 1-Up Mushroom appears above a platform, in between two coins. More cogs with Burners on them are seen, and a Dry Bones is found on a platform. A cog with rotating Burners appears, followed by a cog with four Burners on it. A group of Dry Bones is found, followed by the door leading to Boom Boom. To the right of the door, there is an invisible ? Block with a Fire Flower.
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