The music is a little too quiet, I didn't realize there was a soundtrack until my second playthrough. I also didn't even notice there was a tunnel the first time, and discovered it by accident the second. The powerups are cool and fun, and so is the game. I like it, 5/4/5/5.
PIXEL ART NERD ADVICE
Darker or higher contrast art is better suited to areas that the player cannot go because of how we see images and environments, and low contrast or brighter art is better suited for places that can be traversed to. You should shift two of three from brightness, contrast and saturation to show the difference. Our brains process closed shapes as important, and so lots of little closed shapes can also help to show that something is there vs. not there.