Nice puzzle game coming from a puzzle game fan! Here are my comments.
> I like the concept because it's simple and yet has the potential for a lot of challenging puzzles. Reminds me of Sokoban where there's very few elements but the possibilities for really devious puzzles are there.
> I like that there's an undo button when you hit a spike. This might seem like a minor thing, but it shows that you really had the user experience nailed. As a puzzle gamer you don't want to have to repeat an entire level again just because you mispressed and got hit by a spike.
> The progression of the levels is good (and the level design itself). It lulls you into thinking that it is a simple game but progresses into something very complex.
> Sound / music would be awesome, but seeing as you did this without an engine and eveything, it's very understandable.
> The color hurts the eyes a bit if played for too long. Maybe better if you use lighter ones?
> Not sure how well it fits in the theme, because a puzzle game does not necessarily need to have movement in the first place... so it kinda turned out to be just a normal puzzle game. Great game nonetheless!