I love this art style and the gameplay. I hope you plan to expand this out into a complete game. I'll be keeping an eye on it because what you already have here is so good. Congratulations on a job well done. 

One thing I did notice about balancing:

By purchasing the Laser Turret Level 4 for right at the start the game becomes to easy. I was able to play through the first 6 levels just by saving up enough for one of those, and then one hit killing everything on the board. Once the energy boost comes into play, all it took was 4 of those and one turret to automaticlaly guarentee a win. 

Awesome !