My thoughts after playing your game:
Good
- The art style and ambiance is excellent
- Decent UI, missing elements such as 'how many crystals I collected total/how many needed.'
- Reversed tower defense was a solid theme hit
- Good controls
Area of improvement
- The game is too easy, the challenge is shallow, making me feel bored fast.
- Redundant game elements such as upgrading UFOs (also caused by the last point)
- Poor variety across levels