From the Super Mario Wiki
Grinding-Stone Tower, or Rock-Candy Mines-Tower, is the first Tower of Rock-Candy Mines in New Super Mario Bros. U. This level is unlocked via completion of Porcupuffer Falls. Defeating the Boom Boom in this course unlocks a passage to Waddlewing's Nest and Light Blocks, Dark Tower, and the tower's secret exit unlocks Thrilling Spine Coaster.
The level begins below a platform with a ? Block and a nearby pipe spawning Grrrols. A Bony Beetle is found, along with two Warp Pipes that move upwards and downwards, allowing the player to reach the next platform. More Grrrols are spawned nearby, and a platform with a Bony Beetle and two Brick Blocks is found. A moving Warp Pipe is found, and a hidden opening next to it allows the player to reach a ? Block containing a Mini Mushroom. An opening contain coins is found, allowing the player to avoid nearby Grrrols. After passing by the Grrrols, another sloped platform with a Bony Beetle on it is found, followed by two moving Warp Pipes. A series of platforms with Dai Gorō on them is found, leading to the Checkpoint Flag. A group of Warp Pipes is found, with a ? Block above it and Grrrols spawning from the right. More sloped platforms and Grrrols are found, and a hidden Red Ring is found in a wall. After another platform, two ? Blocks are found, followed by multiple moving Warp Pipes acting as platforms and Grrrols. Two Dai Gorō are found, followed by a platform with a Bony Beetle and the boss door on it.
A Boom Boom makes his species' last appearance as the boss of the tower. Magikoopa swoops in and cause the Boom Boom's arms to turn into wings, which allows him to fly and dive into the player.
Immediately after the third Star Coin is a Warp Pipe that leads to the secret exit. It opens the path to Thrilling Spine Coaster.
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