I got to Wave 9 at my best!
A good upgrade based defense game!
I found that in order to succeed I had to get specific upgrades everytime, and that if I ever fell behind it would lead to a downward spiral. For instance, if I didn't get two Fire Rate upgrades and one Damage upgrade after Wave 1 I was always overwhelmed in Wave 2. It may just have been a skill issue on my part, but I found if I didn't stick to a specific strategy 'tech-tree' approach I'd make no progress.