Well..
First the upgrades kind of suck. For the amount of cash that you throw into bullet upgrades they seem to do very little in the way of damage increases. I had to upgrade to the 5th or 6th level of bullet (3200) to destroy the most basic enemy in one shot. That most basic enemy which took two shots with the original ammunition.
Bullet speed change was almost un-noticeable, again given the price of the upgrades as you went through the levels.
The walls are retardedly small.
The price of both walls and life seemed to change at random, rising and falling without any purchases being made on those lines. And again the value of increasing your life by 1 point at a time is pretty bad.
As well given the amount of money in the game the repair costs were insanely high, and you couldn't do partial repairs. Which also sucked.
Beyond the upgrades, there is apparently a hit detection problem, as there were times I had bullets fly directly over an enemy unit and miss. On the same issue I also saw enemy shot go directly over a wall (not very useful). There are also issues with firing, The turret was making firing noises 2-3 times as often as bullets were actually shown flying out the end.
Furthermore, due to tracking issues with your weapon every enemy shot is a hit, and I felt lucky when I actually got hits in, the enemy tanks don't move very fast, there should be no need to lead your shots.
I think this could have been a great deal better, but I can't give this a good score for what it could have been. Two stars for effort.