"P" Is For Peril: A063-0000-005E-5504
A gimmick level with no enemies. Instead, all the challenge comes from getting through a field of blocks and coins, your only tools a Mushroom and respawning P-Switches, suspended over a pit. Be careful lest an expiring timer dump you directly into the void. My favorite thing about this level is how the player's own actions can remove the very platforms he'll need to stay alive. Some effort has gone into trying to prevent the player from creating situations he won't be able to escape from, but of course with all these sorts of levels there might be some way I couldn't account for.
This is the second take on the idea. The first one was twice as long and had fewer P-Switch respawns, and required the player run back and forth between the start and the end of his range, where he'd try to chip away at a few more coins than last time. I'm still not satisfied with the current version and may revisit the concept eventually. P-Switches are something I'm amazed Nintendo hasn't done more with. I mean, they're pretty much tailor made for puzzle levels!