This is the final level of World One. Warning: lava kills with one hit in this game, regardless of power-ups, so be extra careful!
Start by jumping to the next platform and then grabbing the rope. You will need to climb across the lava using it. Wait for the spikes to move away before advancing. When you reach the end, hop down to the ? block and jump into it to get a Super Leaf. Next is another rope segment, where you have to dodge jumping fireball creatures called "Podoboos." You'll pass over a Star Coin sitting above the lava. Wait until both platforms in the vicinity are raised so you can jump down to get it and then jump back up to the rope.
When you reach the end of the rope, jump down and walk over to the next section. You'll have to leap onto rising and falling platforms in the lava. Take your time, and wait for the appropriate platforms to be at the appropriate elevations before jumping. Also, beware the Podoboos; they can catch you by surprise. Alternatively, you can jump above the blocks at the beginning to reveal some hidden blocks. To the left will be a passageway containing a Super Leaf block. Use the space available to gain momentum and fly over to the second star coin to the far right.
At the end of the platforms is the midpoint flag, which will return you to full size if you are hurt. If you need further protection or have to start the second half of the level over, the first block you'll find contains a mushroom, and the second becomes a Gold Brick.
Swing across on the rope by jumping onto and off of it. Next is a section with Whomps, stone enemies that fall from the ceiling. Wait until they fall down and then hurry through while they're rising back up. Grab onto the swinging rope when the platform you end up on is high enough, and grab a Super Leaf from the ? block on the other side. Jump over another whomp and you will find a giant sized whomp in front of you. Wait for it to crush the ground beneath it a couple of times to grab the third and final star coin of the stage.
Finally, you'll reach a pair of big doors. Head to the far right part of the hall and jump up to reveal a hidden block with a Super Leaf inside. Then, it's time to go in and face the boss.
Boss: Roy Koopa[edit]
In this game, the boss fights consist of Bowser and his various children, the Koopalings. Roy will mostly just charge at Mario until he hits, causing damage. He can also shoot purple fire from his wand, which has to be jumped over. In return, you can hurt him by jumping on his head, particularly when he's still. When hit, however, Roy makes the walls in the room come closer together, and becomes an invincible spiky shell of death. Wall jump to avoid him, and wait until you can jump on him again. A total of three jumps is necessary to win.
When you're done, grab the key that appears above. Congratulations, you beat the first world!