First buy machine gun until you raise enough money for the cannon.Upgrade cannon to full.Remember also to upgrade with points the skills(damage+cannon).You can also buy afterwards shock tower.
It gets very boring, absolutely nothing new just jumping/switching.The graphics are good but the rest its just another uninspired game. Sorry but no more than 2/5.
I think i know why it doesn't work.Cause its not your game.The code its stolen.The code is intellectual property of Walter Reid .Please specify somewhere this important stuff.
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