Great game! It's really satisfying. There are however a few things I'd like to mention.
First, make the walls visible and also let the enemies collide with them. Sometimes I wondered why I can't go right even though there are enemies. Also, warn the player if he is low health. It happened to me a lot of times that I was shooting and suddenly the death count down was seen. Make a warning noise if the player has low health and tint the healthbar red.
Also, please add a score counter so the player wants to improve more and more if he died.
Other than that, it's really a great game and I just couldn't stop playing because it was SOOO satisfying!