Fun game, I made it to wave 30 and then decided to stop. I thought the game was well balanced for the first 25 waves. It didn't feel too hard or too easy. My strategy was to buy both damage and speed upgrades equally, as well as keeping a little 'base' to protect myself. I really enjoyed the building mechanic of the game, I thought it was interesting.
If you work on the game more, I'd love to see different variants of the shooter enemy.
Here's the base I made each round.
