New Super Mario Bros./World 7-Castle

Just when you thought that the bottomless peril is over, the World 7 Castle turns out to be missing entire sections of the floor! You'll have to use Snake Blocks to traverse these huge abysses. The blocks have a mind of their own, so if you don't stay on their odd path, you might find yourself falling to your death. Your opponent at the end of this castle is quite electrifying, and dangerously quick as well!

Level walkthrough
Jump across the gaps and up the steps, then fall on to the green Snake Blocks to begin your journey. As the Snake Blocks approach two Amps, they will start zigzagging upward, creating steps you must jump on. Jump over the first Amp as the Snake Blocks pass below it. The platform will then bend and travel in the opposite direction. If you're quick, you can jump up from the corner where the blocks bend for the first Star Coin. As the Snake Blocks wind around the Amps, you'll also have to jump and duck to evade them.

Next, the Snake Blocks will travel over a bed of spikes. There are multiple spiked blocks that the platform will travel under, as well as some Thwomps that slam down from the ceiling. Simply jump over the spiked blocks to dodge them. For the Thwomps, stay at the back of the platform when it passes below one, then quickly dash across. After the floor of spikes, the Snake Block will arch downward to cross under a spiked ceiling. Then, it'll carry you to a stone platform. Get off the Snake Blocks and go right to trigger the checkpoint.

Now, board the Snake Blocks for the second half of the level. You will have to face several Ball 'n' Chains, spiked balls that rotate around a single block. Jump over the first Ball 'n' Chain, then dodge under the second one. Stay on the rightmost Snake Block as it starts to rise and follow it left. Weave around the same Ball 'n' Chain as the Snake Blocks circle it and move toward the next Ball 'n' Chain. Jump or duck under this one as well as the one to the right. As the blocks start snaking downward, fall on to the block of the Ball 'n' Chain below for the second Star Coin. Jump back on to the Snake Block as it approaches from the left.

After that, the block will pass by some Fire Jets. There is no way to jump over them, so simply stop and hope the jets stop spewing fire before you're forced off the platform. The Snake Blocks will pass over another bed of spikes while still in the Fire Jet section. Falling off is still not a good thing, but the floor at least saves you from instant death if you're forced off, as long as you have a power-up to lose. The Snake Blocks will go past two more Fire Jets before reaching its ultimate destination, a platform with the boss door. For the third Star Coin hanging just off the edge, you must wait at the very end of the Snake Blocks as they start falling. Once you fall with the last block, immediately jump to get the coin and reach the platform in one motion. Go into the double doors to challenge the boss.

Boss: Lakithunder
Once you jump to the top of the castle's spire, you'll arrive at the boss arena, a flat platform high in the air. Bowser Jr. then summons Lakithunder, a Lakitu riding a mean thundercloud. Fitting its name and appearance, Lakithunder can strike the ground with a powerful electric bolt, although it can be evaded if you keep moving around. Lakithunder will also pelt Spinies at you like other Lakitus, although it tosses them more diagonally instead of just dropping them on the ground. Lakithunder can only be hit whenever he swoops in close to the ground, when you can jump on it. Stomp on it three times to defeat it, or just throw a bunch of fireballs at it, netting you the World 8 key!

Note: there will be a tiny pit before grabbing the key, don't fall into it or you'll have to redo the level.

Star Coins

 * 1) This coin is floating near the air above the first set of spikes. Simply jump up when the Snake Blocks are near it to obtain the Star Coin.
 * 2) The final swinging Ball 'n' Chain has a Star Coin on the block it's circling around on.
 * 3) The last Star Coin is just off the edge of the final platform. Wait until you're falling with the Snake Blocks, then jump on to the platform to get it.