The game is built on the standard principle of sequential passing of level levels at one of the three levels of complexity. And the closer you are getting to the end, the more difficult areas for defense and strong enemies are offered. As for the aliens, each of the 13 types of Creep has unique features and style of behavior: for example, some just stupidly rush to the goal, others become invisible to the guns and quickly slip through most of the outposts. So before each new wave of the attack, you will have to partially or completely rebuild the defense systems.
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