I really enjoyed your game! The idea of using randomly-dropped crystals to improve towers is cool, and I loved that you can move the towers around—it adds a fun strategic layer to the gameplay. The first few waves had a bit of a challenge, but after that, it became too easy since I had an excess of resources.
One thing that could improve the experience is better readability of the game state. UX features like showing tower levels, how much damage each tower is dealing, and the exact effects of the assigned crystals would be great additions.
I also noticed a small bug—when I clicked "Buy tower" and then right-clicked to cancel, I still lost some resources. Overall, though, I had fun, and I’m excited to see how the game evolves, in case you plan to continue development! :)