Thanks for the feedback guys! It is indeed a very short level. I was thinking of posting it on my Short/Experimental levels thread since it's so short (it was made in about 4-5 days or so), but I felt that thread's better suited for my unconventional/weird stuff.
Yep, all 100 items should be in there (I confirmed this with a quick test playthrough right before I released the level). However, just as it so happens with pretty much every level featuring a big number of collectibles, it's very easy to skip just one or two and then have a hard time finding whatever you missed
It's usually the ones that are kinda hard to see (e.g. placed over a light orb) or the ones that are in plain sight but can't be collected when you first notice them because you don't have the necessary abilities yet.
Аnd by the way I'm sure I've seen this level before. Only it was darker, and it seems to be a secret level inside another level. But I'm not sure.
Hmm... I don't think so, at least not from my own levels. It should be completely new stuff... unless I have inadvertedly plagiarized someone else's creation, which I really hope is not the case here
Perhaps the idea reminds you of
Reditus' final secret area?