I suggest the number of stars depends on the points for each level. At lvl 75 i needed >4000 points to get 3 Stars. For maximum points you need to be fast, kill as much as possible, upgrade as much as possible and fill everything up as high as possible.
Think this game itself is not a real challenge. But i love to beat others in Highscore.
It´s fun and relaxing, so thank you for this piece of Software-Art :)
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