Never go over Level 30 without them, or you can expect a Leviathan coming for you. Also if your in need of them, and don't have the credits? Build only on Asteroids as they build faster, and also are much cheaper. If they are not built in time of a levi, expect a perma occupation.
Requirements
Credits: 50,000(50% on Moons, 25% on Asteroids) Weapon: Photon Power: 2048 Armor: 1024 Shield: 12 Energy: -24