My strategy was to first upgrade my accuracy a lot and then damage and then other stats. Allowing the player to control the difficulty of the game in such a dynamic way is interesting for sure. I was thinking that it would be interesting if whenever you give yourself an upgrade, the red guys would get a random free upgrade too. That way you would need to readjust your strategy on the fly (if they get a regeneration upgrade, you could counter with giving yourself a damage upgrade. Or if they get a spawn rate upgrade you could counter with fire rate etc.) Though ya a very nice idea overall!