Well, from what I have played a bit, your level seems fairly simple and not too complicate. That's cool.
Some stuff I'd note:
- The background is a bit too bright. Darken it.
- There's not very different enemies aside from the basic ones, like shelless koopas and goombas. Think on adding a different/unique one (usually the ones who goes in SP4). Only a single one is good for your level purpose.
- The green water is a little weird. Maybe you should hue shift a bit to blue, so it looks like turquoise. Also add some basic animation on it, like flowing animation. Specially because you can hear flowing water in the music you picked, so it makes totally sense to make that water flow too.
- Another thing I had in my mind but I forgot.
So yeah, you still have lot of time to work on so I believe with some small tweaks it can be a fun and simple ghost(? where?) house level. Good luck!
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