A very cool way to utilize the theme in a puzzle/tower defense game! There were a couple levels I had to think about how to solve it. My favorite aspect is that the puzzle has two phases: a sort of planning phase, but also an active phase where you have to do things as the wave progresses.
I think with some tweaking the way you can manipulate the crystals could be even better. There were times when I knew what I wanted to do but had to get my positioning just right lest the crystals all crowd around each other again, disabling them. Fantastic entry overall!