Very very good... maybe too easy?
Very very good... one of the best strategy games I've ever played!
However, I had to quit at level 33 because the difficulty did not progress quickly
enough. I found that after purchasing the first few upgrades, the difficulty in the game did not keep up with my earnings and upgrades.
I'd give the perfect 10 for a slightly harder game.
Possibly consider adding more mobilized vehicles throughout, and at the end of some waves, instead of having only soldiers to easily disperse of.
Or maybe reduce the amount of money given per kill.
Otherwise great job!