Nice game and I like how you handled the theme. Balancing the ammount of enemies on screen and their speed was a clever idea. Sound and graphics are nice too. The player felt nice to control.
What I think could be added is a scoring system. I would have liked to see how many kills I had after I died.
I noticed the shots didn't go excactly where my mouse was aimed (they went a little more to the right). Could be an unintentional bug or maybe something on my part?
Overall nice game :)