The bosses are nice, each boss is a bit different than the others, the art is pretty simple, the music and sound are nice. I like the damage increase upgrade where you will use more energy to balance it. Also the tower mode is cool.
Tho you can only shoot in the direction you facing is a bit annoying, and you cant move to the position of the forth boss even after it teleported to another position, I assume you have four boss in each direction and just set the sprite invisible?
But still, nice game!