Great Job
I like the game, it's fun and hard to get frustrated with it. It's a bit too easy though, I agree that more enemies should be added per wave and the upgrade system should be a little more complex. Some good additions to the upgrade system would be recharge rate, traps, and additional weapons. The more challenge you make the game the more you can add to the upgrade system. Get creative and make it a little more unique. Good game, I look forward to the sequel.