Hard game! I managed to finish 2.5 levels I think. :)
Very nice shadow effect! Maybe it should have been 1 extra pixel in size, there were some artifacts on some objects where the shadow didn't quite cover the whole object. There was also a lot of flimmer, not sure what caused that.
Doorways should have been slightly bigger. I could hardly squeeze through! Also, the deceleration might have caused it. Maybe acceleration is enough, and deceleration does not give better feeling to controls?
The hidden areas were cool too, or maybe just unlit areas. The music was very nice, I got Tool vibes from it :)
The dash sound effect should have been high pass filtered. It kind of booomed in my headphones! I think in general it's a good idea to just straight off cut out anything below 100Hz (or maybe not if you make cannon ball sounds or something).
After fininshing the first 3 enemies and finding some hearts it went much better. Could the enemies perhaps have avoided stacking up? There was no blinking to show me that I hit one of them when they were stacked.
Nice submission, good job! :)