New Super Mario Bros./World 1-Tower

The towers in each world serve as major checkpoints, as they let you save your game after their completion. Bowser Jr. awaits at the end if each tower, attempting to take you down himself. Your first tower level is filled with deadly obstacles, stone blocks that instantly kill you if you're crushed by them.

Level walkthrough
You start off on a moving stone block. Let it shift down so you can grab some coins obscured by it. As you begin your climb, be careful of the Dry Bones on the platform. These reanimated Koopa skeletons populate the interior of Bowser Jr.'s twisted structures and they can piece themselves back together after you stomp them. They are even immune to fireballs, so only the elusive Mega Mushroom, Star or Blue Shell can let you defeat these undead creatures. The left of the first set of blocks gives you a power-up when hit. Up above is the first of this stage's stone block pairs. Getting caught between both stone blocks is lethal, so jump over them before they start moving inward toward each other.

The next few set of blocks are a bit thinner, but the gaps between them are not completely aligned with the middle. There are plenty of coins to be collected between each set of stone blocks. Eventually, you will reach a more complicated set of moving blocks. Take your time to observe their movement before trying to get across them, since you will have to past it multiple times. Past the stone block combination, hit the first block for another power-up. The second block contains multiple coins, and to the right of it is the first Star Coin. Go back to the left and continue your ascent. Here, wait for each stone block to extend to form a staircase that you can jump up with.

Now you can pick between two paths, one with a Star Coin while the other is quicker and easier. Go up the right path and jump onto each stone block as they extend to the left, being careful not to accidentally wall jump into the spike blocks directly to your left. Past the spike-block section you can get the second Star Coin, followed by more stone blocks that can potentially crush you. Jump as soon as the stone blocks start to come together so that you land as the blocks form a platform. Do this two more times to clear the difficult right path.

If you want to skip the second Star Coin, go left and duck into the green cannon pipe. Instead of transporting you elsewhere, this pipe will launch you up along a trail of outlines, making coins appear within them. Let yourself fall back into them to collect all of the coins. Launch yourself up again and drift to the right a bit this time to land on a platform with a door. Go through the door to enter a smaller room with a bunch of erratically moving platforms, as well as the final Star Coin suspended in the air. Go through the door to return near the cannon pipe. Launch yourself back up and continue climbing up the tower to reach another cannon pipe. Now, you must time your launch to avoid getting crushed by the stone blocks above. At the top, bump into the block for a power-up before going through the double doors, which takes you to the boss arena.

Boss: Bowser Jr.
Bowser Jr. is not giving up Princess Peach without a fight. Your first encounter with this Koopa brat is very simple. Bowser Jr. will only try to hurt you by running into you. You must stomp him as he gets near to hurt him, after which he will duck into his shell briefly, becoming immune to stomps and being capable of hurting you if you try to jump on him (the hint of the danger is a brief flash of red when he ducks into his shell). Three normal stomps are all it takes to defeat Bowser Jr. A hit with a Ground Pound counts as two stomps. Alternatively, you can simply pelt him with eight fireballs as Fire Mario.

Upon defeat, Bowser Jr. will flee to the World 1 castle with Peach. Keep up your pursuit!

Star Coins

 * 1) The first coin is near a set of moving blocks. You can get it without too much difficulty, but watch out for the Dry Bones nearby.
 * 2) At the first cannon pipe, take the harder right path to get to the second Star Coin.
 * 3) After launching yourself from the first cannon pipe, fall slightly to the right to reach a door. Go through it to reach a room with the Star Coin.

Alternate exit
For this exit, you will need the uncommon Blue Shell item, which can't be found in levels. You must get it from one of the flying blocks visible from the overworld, or from a red Toad House. Make your way to the room with the third Star Coin. Among the many shifting stone blocks in this room's border, there is a mysterious Mario-sized space that remains open at the upper-right corner. Jump to that spot and go right to enter a secret area. From where you end up, jump around to make some hidden blocks appear, one that contains a 1-Up. If you aren't Shell Mario, you won't be able to progress any further and must go through the yellow pipe to return to the main stage. If you are, dash until you duck into your shell, letting you break through the blocks beneath the yellow pipe. Keep going to break past the wall of blocks covering the red pipe. Enter it to get yourself deposited outside the tower, where the red flagpole is.

This secret exit opens up a path to the World 1 Cannon, which launches you far into the chilly plains of World 5. You might not want to take that big of a shortcut if you're inexperienced, as the spike in difficulty is quite significant.