I've get all upgrades on wave 15, or even earlier, so there is nothing to do further, last 5 waves was all -the - same repetetive shit. ( Have 2 - 3 new types of enemies and weapons would be nice. Difficulty becomes to low, when you get damage + accuracy maximum upgrade.
Ideas for new enemies :
1. Enemy snipers that shoot you, not your base, after short amount of seconds. They come from the top of the hill, not from the left.
2. Barely visible scouts in camo, that tries to sneak into your base and explode it, from inside.
P.S. Enjoyed the game a lot, at first 10 waves ) nice job