The mix of the theme with a tower defense was interesting... You were able to make a lot of content with simple ideas. The main menu animation was nice too.
But the tower management was a little confusing... The UI didn't help in understanding what was going on... The bar would change color and the tower became faster when I let it go of her... Didn't made much sense...
it would be clearer if you made the tower recharge automatically when the player wasn't shooting... And just showed the bar growing with the energy recharging.
The fonts on the tutorial didn't help either... They were a little hard to read... And there was a lot of information smashed together... Try mixing up the texts with some visuals... You can use the game sprites... or simply make a level that is playable to be the tutorial...