This level has you climbing a tower. You will face new skeletal versions of old foes, including goombas. Stomp the one above you and hit the wide ? block to get a Super Leaf. Continue climbing up the moving green blocks while avoiding the Piranha Plant. Use the diagonally aligned platforms to continue going upwards. For the last jump, wait until the green blocks give you a bit of extra height. When you reach the area with the Red Ring, stay on the left and Wall Jump to reach the first Star Coin. You should easily be able to reach the midpoint flag.
Above the doorway, which is best explored later, there will be a bunch of moving green blocks. To the left is a red pipe. When you go in it, you will see a P switch and moving green blocks. Hit the switch, but DON'T go after the coins. Instead, use the blocks to ascend to the second level, and jump on top of the bricks to get the second Star Coin. Then, exit to the right.
You should be directly below the final Star Coin, but will have to do some navigating to get there. Jump on the green bricks to the right and use them to climb up, avoiding the enemies along the way. Jump over the Bone Piranha Plant to the right of the coin and wait until the bricks move, dropping you on the third and final Star Coin. Quickly jump back up to land on the moving bricks in time and avoid being hurt.
Finally, go to the platform to the upper right, and jump onto the new set of moving green blocks. These will act as an elevator, and bring you up to a big set of doors. Inside is the miniboss for this stage, a pair of Reznors. These can be defeated by simply jumping up into the blocks they're sitting upon.
Secret Exit[edit]
Above the midpoint flag, there is a door. In it, you will see a pipe spawning bone goombas green blocks moving into a hole to the right. When the way is clear, go through the hole and into the pipe inside. You will find a flagpole, and the way to World 1's Cannon will be clear.