It's a good game and a good idea, well done!
The only thing I didn't like is the fact that movement is unnecessary: since enemies spawn from only one direction, you can go the other direction and shoot them peacefully, you can fix that by adding collectables objects (coins), or letting enemies spawn from all directions.
Anyway, good job! And please check out my 2 submissions: Speedrun Unlocked ! and Escape the Bug.