Bowser Jr. has spent some time installing conveyor belts in his World 5 stronghold. With many Piranha Plants catching a ride on these belts, you will have a difficult time clearing this level without a Fire Flower. The deeper parts of the castle have frozen over, forming icy surfaces in addition to the conveyor belts. The boss of World 5 isn't ice-themed, but fans of the series are sure to recognize him.
If it wasn't immediately obvious upon entering the level, this castle's main hazard is conveyor belts. The first one moves you to the right and is not much of a worry. However, the ones after the gap mostly move you to the left, making it troublesome to progress forward. Additionally, there are Piranha Plants on the conveyor belts. Without a Fire Flower, you must jump at the right time to dodge them. There is another Piranha Plant that will fall on to the third belt from a conveyor belt above.
The first Star Coin is coming up soon. Jump to the right and you should see the Star Coin on a stone column. You must fall to the column to collect it, then quickly get out, as a Piranha Plant is also getting transported on to that column from the conveyor belts. After another conveyor belt, you will have to jump across a series of stone columns with Dry Bones on them. Jump from those columns on to a conveyor belt, then to another stone floor. There is a menacing Venus Fire Trap on the conveyor belt to the right. The movement of the belts let it shoot plenty of fireballs at you before falling into the abyss.
Go down under the stone wall using the conveyor belt. On the other side, conveyor belts carry some Dry Bones and a single Big Dry Bones. It will also transport a spring to you. You can use that spring to launch yourself high up. Use this bounce to reach a ledge on your left, which contains the second Star Coin. Go along some more conveyor belts, avoiding some more Dry Bones and Piranha Plants. Once you pass wall made of ice bricks, you're near the checkpoint. Jump over another conveyor belt and on to the icy surface to activate it.
From the checkpoint, there are several conveyor belts depositing Piranha Plants on to the ground. You may wait for the Piranha Plants to get dumped before using the belts. There happens to be a roulette block at the center of the third conveyor belt from the checkpoint, but watch out for the stationary Piranha Plant nearby. Up next, a Super Piranha Plant will be deposited on to a portion of the floor, creating a huge obstacle without a Fire Flower. Use both conveyor belts above it to jump over the monstrosity. After the Super Piranha Plant is a single block, with a one-tile wide gap above it. Get on the block and jump up into the gap to get to the ceiling. Run right and fall down the shaft, then go left along the belt to reach the third Star Coin. Jump through the thin platform to get to the double doors leading to the boss.
Boss: Petey Piranha
If you're wondering about the reason for the abundance of Piranha Plants in the castle, here's your answer. Petey Piranha allied with Bowser Jr. once before, and he is back for revenge. Petey Piranha attacks similar to how he did in Super Mario Sunshine, flying up high and trying to land on you. He will slip when he lands, giving you the chance to stomp him. Three stomps or several fireballs are all it takes to defeat this overgrown plant once again.
You may have noticed that World 5 leads to two other worlds, similar to World 2. Indeed, beating Petey Piranha normally will let you go to World 6. However, if you beat him in the same secret method as you did with the Mummipokey, (that is, while tiny), you can unlock World 7 instead. The fight is mostly the same as Mini Mario, only you need to ground pound instead of just stomping to harm Petey. Without any blocks that contain a Mini Mushroom, you'll need to get one from a level or Toad House elsewhere. The roulette block in the latter half of the castle can potentially give you one as well.
- The Star Coin can be seen above a stone pillar and isn't hard to get. A Piranha Plant will be deposited nearby if you're not quick enough.
- One can see the Star Coin on a ledge high above the ground. Wait until a spring passes by on the conveyor belts. Use the spring to launch up to the ledge.
- After the Super Piranha Plant is a single block with a small space in the ceiling directly above it. Jump on to the block, then jump through the gap and on to the ceiling. Go right until you fall down a shaft, then follow the conveyor belt to the left, which leads to the Star Coin.