New Super Mario Bros./World 4-Tower

Despite his best efforts, Bowser Jr. couldn't find a way to incorporate the jungle's abundant poison water in his tower. However, he has another lethal liquid at his command: lava. Get ready for some quick fence climbing, as you'll need to avoid a constant rising surge of molten rock. Get trapped or fall off, and you might get burnt to a crisp.

Level walkthrough
You can see lava covering the floor as soon as you enter the tower, but this isn't the main threat in this starting section. That would be the series of stone blocks that serve as stairs to ascend. Time your jumps as each block close in below you to hop up the blocks one by one until you reach the top. The two blocks here contain a power-up and 10 coins, so it's worth your time to bump into them. Enjoy this period of serenity, as the pace of the course will radically change once you go through the door.

Once you exit the door, the primary hazard of this tower becomes immediately obvious. The pool of lava below will start to rise, forcing you to begin your ascent up the tower. There is a large fence just above you that you must use to climb up. There are flipping gates on this fence, as well as many other fences in this level, allowing you to switch between both sides of the fence. For purposes of completion, you're going to want to switch and stay at the back of the fences. Jump on to more fences until you get to one narrow, tall fence. Jump to the ledge to the left. Go pass the moving stone block and collect the first Star Coin placed behind the fence.

Jump on the fence above the Star Coin and keep climbing up. Hop on to the fence covering the width of the screen and wait for an opening to form between the two stone blocks directly above. Jump to the fence above and quickly climb to the right. The second Star Coin is below the ceiling, surrounded by Amps. Leap to it and fall back to the fence. Swiftly move to the left, then jump to another vertical fence. Leap right on to some steps, and dodge the spiked stone blocks as you continue your ascent. Up another fence and go left to pass by the checkpoint flag.

There are plenty of moving stone blocks here, so the chance of getting crushed is higher. There are two paths you can take in this section. The left one is narrow and straightforward, while the right path is wider and a bit of a detour from the main path upward. You want to go right, passing the Dry Bones, and jump up these long, thin fences. At the top is the third Star Coin below a dead end. With the lava quickly approaching, you can get on the closest stone block and duck as it moves left for a shortcut back to the left path.

Up one more fence and there will be no more moving stone blocks. The final stretch of the tower is devoid of enemies or crushing obstacles, but the lava is still soaring. You must scale up to the double doors using only fences. For an additional challenge, you can activate the red ring here to spawn some red coins. The red coins are all suspended between fences, and the path they direct you in is not the fastest way up. You might accidentally fall if you make a misstep while collecting them, so just skip these coins if the lava is worryingly close. The lava will stop once you get to the double doors. Enter it for another Bowser Jr. rematch.

Boss: Bowser Jr.
You'll notice Bowser Jr. is now wearing his mask. He's finally mixing up his attack pattern, but that doesn't make him much harder to beat. He is much more defensive this time around, ducking into his shell whenever you get close. His main form of attack is to throw green Koopa shells at you, which is also the primary way to harm Bowser Jr. Whenever he throws a shell, jump on it and kick it back at Bowser Jr. to knock him down, which lets you stomp him. Three stomps defeats Bowser Jr. as usual, and fireballs are just as effective as in any other battle with him. Notably, there are two small pits which are used to remove any shells that rolled off the main platform. You can fall into them and die, but this is very unlikely.

Star Coins

 * 1) Make sure you are on the backside of a fence before seeing any Star Coins. Once you see the first moving stone block in the rising lava section, Go left past it. The coin is behind a fence.
 * 2) Soon after the first Star Coin, stay on the right side of the screen. You can see the coin soon, and you must jump from the fence below between the Amps to get it.
 * 3) Stay on the right side of the screen as you encounter some horizontal moving stone blocks. The Star Coin is at a vertical dead end. Duck on the nearby stone block once you collect it to make a quick escape.