Nice submission for your first.
Liked your take on the theme and game design.
Some suggestions to improve the game:
1. Zoom out a little - very hard to see where the enemies are coming from
2. Modify your shooting mechanic - no reason to use user constantly click. Holding down mouse should keep shooting. A suggestion I wish I would listen to more myself: Place yourself into the shoes of the game player and make sure the game is simple and easy to understand and not frustrating [at least of the easy difficulty].
3. Sound/Music really adds to the feel of the game (I often leave this too late as well).
Let me end with this: You have a good start here and again, this is a very good submission for your first, keep with it and your submissions will continue to improve