New Super Mario Bros. U: Rock-Candy Mines- Castle -- Roy's Conveyor Castle

The castle is filled with conveyor belts carrying large blocks which can crush you. Some of the blocks form stepping stones in the lava. When you reach solid ground, you'll see the first Star Coin under a ledge. A set of three blocks will fall. Let the first two fall, but run before the third falls because it will block access to the Star Coin. There are conveyor belts under the lava which allow the blocks to move through the lava in some spots. Ride one of the blocks moving to the right and you'll see gaps in the low ceiling. The first two gaps have coins in them, but the third one does not. Wall jump up the third gap to the hidden pipe above. It leads to a room with the second Star Coin on the right, next to a conveyor belt. Run quickly to get the Star Coin before blocks drop in to cover it. When you return to the main level, there's a Red Coin Ring near the midpoint. Past it is a ? Switch that makes blocks and Dry Bones drop from above. You'll need to hit the switch to make it to the boss door. Don't worry about the third Star Coin just yet. Go through the boss door and launch from the cannon to Roy's Airship. Oddly enough, this battle takes place outside. Roy Koopa Jr. has a bazooka-style Bill Blaster. Flying platforms will appear overhead. Stomp Roy, preferably before he can fire any Bullet Bills. Three stomps will defeat him. This opens the path to Meringue Clouds. To get the third Star Coin, take the path behind the castle. There's another ? Switch that will make blocks fall. Let the first few form a path in the lava, but move quickly and run under the ledge for the third Star Coin before a block blocks your path. Keep going to the left for blocks that will move through the lava to the boss door. You'll have to duck before you reach the boss door. Defeat Roy again. We now have all the Star Coins in the Rock-Candy Mines.