Command Centers (CC) Each Command Center boosts your overall defensive power by ten percent. Veterans will build ten to fifteen CCs on a base to improve the power of the defensive fleet up to 75 percent. Building ten is considered reasonable for a free player. A Heavy Cruiser with 10 CCs and medium plasma research (10-15 levels) will dish out 125 points of damage.
Turrets
These are grouped together, because anything smaller than a Disrupter Turret is useless against Dreadnoughts or higher. Most defenders will keep two of their best turrets until they get to Disrupter Turrets, then they will build 4 or more of the D-Turrets.
The major selling point of the Ion Turrets is the shielding they have, which helps them against unshielded units. Ion is also a cheaper technology to research.
P-Rings
This is not a unit, but a defense. P-rings are the first deterrant to capital ships. Most players get P-Shield tech way before P-Ring tech. While the P-Shield won't protect you from capital ships, they will protect you from the currently rampaging Cruiser and in lesser degree HC fleets.
adapred from http://espacesociety.org/AstroEmpires/SmallFleetDoctrine
