This game is ALMOST perfect!
The concept of infinite dimensions is fine, you could add more "plot" to it, but that's ok.
The game mechanism is flawless, fine amount of enemies and powerups, no problem here.
The art is very simple, but that's a style we call "retro", nothing to blame here then.
Only thing that I disliked is the controls, there should be a mouse option (ship follows cursor), this would make everything more enjoyable, as many people, like me, have more confidence over this device. Or maybe allow you to edit the control settings, it kindapisses me off to have that "\" key between Z and Shift, lol.
Sound is amazing, specially the music, it merges you even more deeply into the game.
In general, you made one of my favorite games overall, good job!