Cool game. It's nice that you focussed on readible enemies and feedback.
I encountered a bug where I cant reach the next level (in the level with the 4 shield guys)
The opening door is not communicated really clear, the small green lights are not really cleary to see. Suggestion; Lock the other ones once the round starts (that will also convey a mmore urgent feeling to the player. And then unlock the next room once its cleared. I miss a rewarding feeling when I clear a stage.