In the Free The Last Shelter Android Game very interesting plot and history. Imagine that somewhere on other planet life was found. So it was decided to arrive there to construct towers, and then the whole colonies to rescue mankind and to transport it to new colonies.
But during construction the colony get under attacks of alien bugs. So your main objective is to strengthen and to protect colonies to help the whole mankind to escape. It will be necessary to place the armed towers and to beat off from attacks of malicious alien aggressors which will disturb the execution of your mission.