I love the retro look you guys went for both visually and sonically, I think you guys really hit the nail on the head with that. I had an issue with enemies going off to the right but it's already been mentioned so you guys already know.
The color boxes to indicate where the enemies are coming from and what color they are works very well.
The onlt thing is that, once I found that I could just hold down the left button of my mouse and drag the cursor from one side of the screen to the other to kill the enemies the game became substantially easier. You kind of lost me there.
It'd be cool to see if you can implement some kind of "kill sttreak rewards" or maybe add some different enemy types as you progress through the game. I am sure it'd make the whole game more engaging.
Overall though, awesome job. Loved the music, the sound and the look and, other than the aforementioned problem with the enemies disappering off screen the game ran well and I had fun so good job!