Security Systems are new defense-only Units that protect your Base and regular Units from attacks (including Missiles). Security Systems will always defend first, to prevent the attack from destroying your other Units.
Different types of Security System Units defend against specified Unit types in attacks on your Base.
Security Systems are available to players who have reached Level 15. You must also have a Defense Core building. Hiring Security System Units requires Munitions, Fuel, and Rations.
To start hiring them, go to the Defense Core and open the Security Systems tab. Tap on a System to select how many you want to hire. You can speed up the hiring process in the Queue tab.