Grinding-Stone 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 course's main obstacles are pipes that move left, right, up, and down and Grrrols and Dai Gorō, who return from Stoneslide Tower.
A Boom Boom makes his species' last appearance as the boss of the tower. Kamek will swoop in and cause the Boom Boom's arms to turn into wings, which allows him to fly and dive into the player.
Make your way up as you avoid the grinding rocks and Dry Beetles. The latter will occasionally bear their spikes and make it dangerous to jump on them. When you see a pipe moving forward and back, get on top of it and go through the wall behind it and it will lead to a Mini Mushroom. Now carefully get past the grinding rocks and run up the wall into the mini-pipe. Get past the giant grinding rock and get Star Coin #1 then run up the wall to get out. Star Coin #2 is located in a hidden wall. What you do is the moment you get out of the pipe and wait for the grinding rock to go to the very left and you can just run up the wall and into the wall with Star Coin #2 and hit the continue flag from there.
Get the mushroom to get big again then hop up safely and go into the wall for the red ring then collect the red coins. Turns out that the grinding stones can be bopped from below. Work your way up as you get through grinding stones and moving pipes. This reminds me so much of a level from Battletoads. Once you make it to the top, go inside the door to face Boom Boom.
Boom Boom sprouts wings just like he did in Super Mario Bros. 3 in some castles. Just stomp on him when he swoops down at you. He gets more vicious with each stomp and he'll die in three hits.
- Dai Gorō
- Bony Beetles
- 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 right. As with the first Star Coin, Mini Mario is required to reach this Star Coin.
Immediately after the third Star Coin is a Warp Pipe that leads to the secret exit. It opens the path to Thrilling Spine Coaster.