Fun game! I liked needing to think about whether to switch to offense or defense once enemies appeared simultaneously.
Good use of color and music made for a nice atmosphere. You made a simple look feel attractive.
I'd like to see a bit more animation on the energy field around the player. It looks really static right now. Maybe some spinning or pulsing would make it more lively. It looks good during the switch between polarities.
I think shooting would feel better if the rate of fire were a little higher and the shot had a more powerful sound effect. You could also put some spin animation on the shots to make them look more energetic.
Strangely, all enemies stop spawning after a while, just when it was starting to get really exciting. :(
Sometimes, I seem to die even though nothing hit me. That's confusing. Does that have something to do with the bar at the top? That bar, whatever it is, requires more explanation.