Alternate level name: Mind the Gap
I have somewhat mixed feelings, but much respect, for this level. It takes an underexplored sprite and abandoned beta item and uses them in really inventive ways. At the same time, it reveals that while SMW's physics are awesome, its interaction is a heaping dumpster fire and works against this level as hard as it can.
I'll start with positives: The layer 3 cage segment was set up extremely well, and absolutely memorable. I like how the line guides thrust the enemies that you have passed back in your direction for another two or three rounds of action and it is done so effectively that while it gets in your way, you have an opportunity to dodge. There is so much apparent misdirection in the lines that you can watch things come in from the outside then, they suddenly move. Parts where the ghost gaps let you through walls are synchronized perfectly, though I did find one occasion where I ended up outside the cage, found that I could walk on air, and I didn't really try to do it a second time. The On/Off line-guided segment is also done well, feels involved but not too tricky. I can't see the originating point of the platform I'm supposed to drop and manipulate, and it caused me to just keep hitting the block, waiting for something to happen. Everything after the midway point shined and I thought the design was cool and crazy. Palettes were well-done. While in the video stills the pastel wood seems like it's receding, there is no question during gameplay at any point that it's interactive. Secret stuff is actually challenging to get. I gave up on trying to figure out how to get the moon in the room that could be reached if a certain secret is found behind the clouds.
Downsides: As NopeContest pointed out, I had multiple issues even moving with the floor of it causing Mario to twitch and glitch to death inside the cement blocks, and for that, I had to rely on savestates. Also, the fact that Mario sticks with layer 1 when the level scrolls causes Mario to slide to the back of the cage all the time. I have had a few issues with floor sprites either disappearing entirely (in the on/off section near the end, but this was actually a blessing)
or not letting me fall through at all (early section with the Grinders and Dragon Coin... I tried resetting the latter by exiting the screen and approaching again but couldn't fall through. (Steps to replicate: many sprites were on screen at once. I had begun attacking Chuck, Banzai Bill spawned, and I think I had 2 Grinders(3?) on screen then the gaps stopped letting me fall through. It's probably a sprite memory problem causing a failure in the floor's functionality.)
(Oops, I was doing it wrong. I just wasn't big enough.) The On/Off room had me lost for a while because I could not see where the platform I was supposed to drop and manipulate originated. I kept hitting the block, waiting for something to happen, and wandered around a bit before it finally came down. I'd move it within view.
Some other thoughts... The level is either very long or feels that way because I got eaten by the wall a dozen times, and I think the first room has a little bit of bloat that could be shaved by reducing the length of sections where a lot of redundancy is there to no enhanced effect, e.g. reducing that whole part with the Grinder and fall-through floor, and the reduction of the number of sprites may also help it work as intended with 100% consistency. It may also help fairness if you stagger rather than vertically-align the ghost floors in that part where there are two gaps through cloud + cement structures, as it's so easy to fall through both when big and it's almost a given to approach it at an angle as it gets ready to turn around. The first room kind of does seem like it's trying to introduce the gimmick, but it lacks the strong focus of the last two rooms. I think given the intensity of the cage section versus the layer 3 cage room, that the layer 3 cage room would make the ideal "finale" for the stage.
Just look above you...
If it's something that can be stopped, then just try to stop it!