*If the Enemy ship disappears after 2 seconds* I had a typo in getNextHighestDepth. So it didn't read that it should assign a new number to each one and was generating the same one over and over again (i.e. deleting it every 60 frames and regenerating it on the left side of the screen).
I liked it. The graphical style of it flowed nicely, appealed to me and fit well with the vibe of the game. If I was forced to make criticism, I'd say the game was slightly on the slow side, lacked replayability and somewhat linear. However, I don't think you were going for any of those things, so... yeah... 5/5. Keep up the good work. (Also, nice ending)