World 1-Castle (New Super Mario Bros.)
From the Super Mario Wiki
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The majority of the level consists of walking on tightropes over lava. Along the way, Mario encounters Lava Bubbles and Dry Bones. There are also several platforms that move up and down, some of which sink into the lava. Eventually, Mario comes to safe ground. However, it is below a ceiling of spikes that slowly lower themselves down to the ground. Mario must go to gaps between the spiked ceilings to avoid getting crushed. The first Star Coin can be found in one of the gaps, and Mario must Wall Jump to reach it. Mario then must get through a second lava-filled area. The second Star Coin can be found on a moving platform that sinks into lava. Before making it to the boss door, Mario can jump on the last tightrope to find a hidden Warp Pipe to get to a room with the final Star Coin.
Bowser is the boss of the first world. The fight is very similar to the ones from Super Mario Bros., as the fight takes place on a small bridge above a pit of lava, with a Skull Switch (which replaces the axe from Super Mario Bros.) at one end. Mario must either go around Bowser and hit the switch, which will collapse the bridge, or defeat him with fireballs. After the Bowser is defeated, he falls into the lava and becomes Dry Bowser, and is not seen again until World 8.