Very interesting idea, with a fantastic aesthetic to boot! However, some of the enemies and their abilities are cheap rather than actually challenging. For instance, on some levels it's impossible to defend from an attack due to a bomb destroying the only shield blocks available (and that's if spawn at all). Usually a few tries would get me through a stage fine, but others just don't allow for the kind of experimentation needed as you have to be on your game from moment one.
The game does do a great job at communicating what is will happen to the player though, each mechanic and obstacle is introduced and explained well. I think with a little tweaking of these mechanics would improve the game exponentially!