Cool game ! really fast & nervous.
The indicator on screen to tell you where the ennemies are coming from is a really nice touch.
Tiny feedbacks :
Having a retry button when you die would help a lot to keep the player longer in the game. The "ok one more try" feeling can go away if you have to go back through the menu.
Also some light screenshake when you fire your weapon would help a lot to add more juice to the game.
Apart from that, it's a pretty solid entry. Good job :)