Good game and good mechanics! Simple, challenging and fun, and lots of to play. This concept has capability to be modified into many directions too. Creativity has been used well in enemy types and level design.
You should add shadows and make units larger. Also enemy types which are different would have different color and/or model shapes. I would like to see the "charge" as in meter, so i would be able to play with no sounds, or it would make the game play more clear. I would like to see some other power ups to be collected than just a hearts - maybe a gun booster for stronger or for a spread shots. Background would be more colorful.
This is cool, so please share to us to us if you decide to build an android version. You should theme this game carefully because this is kinda unique.