New Super Mario Bros./World 1-Castle

Bowser Jr.'s first fortress is already quite impressive, with tricky tightropes and a castle-wide pool of lava. Large moving slabs of stone as well as the aforementioned tightropes are your main method of crossing this deadly hazard, while spike ceilings are installed wherever there isn't lava. Mario's arch-nemesis awaits at the end, ready to put an early end to your adventure.

Level walkthrough
You will come across a tightrope soon after starting the level, just one of many throughout the stage. You never have to worry about falling off the tightropes, and they are often quite beneficial. The tightropes sag whenever you're on them, and you can get a height boost by jumping off of a sagging tightrope. After the tightrope is the first gap with lava. Avoiding the lava is obvious, as it causes instant death upon contact, but you also have to beware of the Podoboo that shoot up regularly. These blobs of fire appear wherever there is lava, and you will encounter a lot of them in this stage alone. After that is another tightrope, leading to a moving stone block. Use the stone platform to reach the block above it, making a power-up pop out. After that, you must cross a stretch of lava by using stone platforms rising and sinking in the lava. The platforms will never completely disappear into the lava, so you can just take your time with crossing them.

Past the moving stone platforms is a long path with ceilings lined with spikes overhead. The ceilings will slowly move down and threaten to impale you, so you want to move across these stretches of spikes quickly. Turquoise blocks on the ground indicate spots safe from the spikes, making it a good spot to hide in if you're not confident you can clear the spikes in time. The second safe area contains a 10-coin block that you can use the ground pound trick on. The safe zones will get smaller and smaller, and the last one is even unmarked, although there is a Star Coin right above it. You can obtain it by wall jumping off the sides of the spiked ceilings when they have fully descended. The general tactic here is to be patient, as the timer is long enough for you to move between safe zones carefully. Once you've cleared all of the spike ceilings, you have reached the checkpoint.

A sea of lava follows the spiked ceilings. Use the tightrope after the checkpoint to reach the block that contains a power-up. Leap onto and across the stone block to a series of tightropes, where a couple of Podoboos can shoot up in awkward spots. From the tightropes, jump to the stone platform with the Dry Bones. Keep the Dry Bones incapacitated and wait for a short stone platform to rise out of the ground to the right. This lets you collect the second Star Coin safely without falling into the lava. After that, jump on to the tightrope. Dodge the Podoboos and jump to the next tightrope, the final and longest one in the course. Get to the middle of the rope and jump up to reach the entrance of a warp pipe just barely above the screen. The pipe takes you to a secret room containing the third Star Coin. Use the warp pipe to drop back onto the tightrope. Jump across the final two stone platforms to get to the double doors.

Boss: Bowser
Bowser Jr. must either be desperate to stop your chase or just haven't planned his kidnapping very well, since he has requested his father to serve as the boss of World 1. It's apparent that the latter is more likely, since Bowser's arena and tactics are rather simple. This boss battle plays out similar to the battles in Super Mario Bros.. Bowser will fight you above a pit of lava, and he can either shoot fireballs or try to jump on you. Your goal is to get to the skull switch at the right end of the bridge, which will cause the bridge to collapse. There is a moving platform at the left that helps you leap over Bowser. Alternatively, throwing a barrage of fireballs at Bowser will cause him to tumble into the lava as well.

The hot lava thoroughly scalds Bowser, searing off his flesh and leaving a skeleton behind. With Bowser out of the picture, Bowser Jr. is now in charge of the entire Koopa Troop, and he isn't going to abandon the kidnapping due to his father's defeat!

Star Coins

 * 1) Between two spike ceilings. Wall jump up to grab it.
 * 2) Suspended above the lava. Wait for a small stone platform to emerge from the lava before collecting it.
 * 3) In a secret area reached via warp pipe. Jump up in the middle of the last tightrope to climb into the pipe.