Kool. Good work
Some suggestions.
1) Pause button
2) Visual feedback when you get hit
3)
go to out about 1.22 seconds ; I think on your game you need to decrease the enemy line length each time the farthest enemy is eliminated. They should move all the way to the right of the screen (if this makes sense)
4) I'm not sure if this is how the game is intented, but it seems to me the enemies move reallly... really fast after a little while. Did you limit fps/take into consideration different computer speeds?
Maybe include an attachment of the source code so people can peek at it