Great idea! I thought the idea and art was great. Here is my feedback:
- As others have mentioned, I didn't know you could upgrade because it was never called to my attention. Making this more obvious would help
- I simply held down the space bar and never let go to have an endless beam of damage. Having some kind of counter-measure here would be great... like a cool-down on the light or a special enemy that is designed to encourage you to stop blasting light in order for them to become vulnerable.
- I got to a point where my speed and damage where too high and creatures would die the moment they touched my light. I was too powerful!
- Upgrades should be more expensive after each purchase in order to pace things out a bit.
All-in-all, great work and fun game :)