This is a defense game where you need to protect your scape colony from the incoming enemies. The enemies appear at the top part of the screen and gradually lower until they reach your base. Your goal is to kill them before they do it.
There are 2 types of structures that can help you with this: Turrets and barracks. Turrets just shoot the enemies that get close and barracks can train soldiers that will do more or less the same: Shoot the enemies.