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Difficulty for the first minute was fine. However, after a few minutes I walked away and forgot about the game with 3 white and 5 reds. When I came back almost two hours later ("Survive" was about 6350) those same towers were still keeping up with no problems. Also, my "DNA" was blank, I filled the entire grid with whites just to see if something would show up, but it never did.
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The instructions or the game should give you the prices for buying and selling. Red=20 White=40 Selling is half back. Other than that, theres no purpose, no goal, no upgrades or advances progression. Once you get 2 white cells right next to the start, you can just leave it on endless. It doesnt seem to ever get harder, and there are no new viruses and they never come faster, more frequently. Pretty big let down all around on this one.
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It would be interesting if you put at least the price of the Red Cell and the Upgrade instructions. Further more types of missions or something like that.