Difficulty: very easy
I was rolling a Doom random level theme generator for speedmapping purposes, and one of the prompts it gave was “The Wreckage”. I didn’t really know how to make that in Doom in only an hour, but I did know how to make it in Mario, so I did.
The additional rules were “no monsters” and “no stairs”, so neither of those things appear in this level. It’s quick and entirely atmospheric. I like it. Though it’d be slightly better if I’d correctly named it “The Wreckage”. Oh well.