Grinding-Stone Tower

Grinding-Stone Tower, or , is the first Tower of Rock-Candy Mines in New Super Mario Bros. U. This level is unlocked via completion of Porcupuffer Falls. Defeating 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.

Layout
The course's main obstacles are pipes that move left, right, up, and down and Grrrols and Mega Grrrols, who return from Stoneslide Tower.

Boom Boom makes his last appearance as the boss of the tower. Kamek will swoop in and cause Boom Boom's arms to turn into wings, which allows him to fly and dive into the player.

Star Coins

 * Star Coin 1: A mini Warp Pipe, that leads to the area with the first Star Coin, is present at the beginning of the course, to the left. Mini Mario is required for this Star Coin.
 * Star Coin 2: Hidden within a wall to the right, after a series of Grrrols.
 * Star Coin 3: Another mini Warp Pipe, that leads to the area with the third Star Coin, is present at the end of the course, to the left. As with the first Star Coin, Mini Mario is required to reach this Star Coin.

Secret Exit
Immediately after the third Star Coin is a Warp Pipe that leads to the secret exit. It opens the path to Thrilling Spine Coaster.

Enemies

 * Grrrols
 * Mega Grrrols
 * Bony Beetles