In Sid Meier's Alpha Centauri (SMAC), the Citizen's Defense Force is a Secret Project. It may be built at a cost of 300 minerals, on discovery of Intellectual Integrity (E3). The Citizen's Defense Force provides a free Perimeter Defense at each of your bases: double defense strength for units in the base. The Perimeter Defense base facility costs 50 minerals, so the Citizen's Defense Force is a bargain at only the cost of 6 Perimeter Defenses.

The Hive faction (blue, Yang) already gets a free Perimeter Defense in each base as part of the faction design, so the only reason to build it when you're playing the Hive faction is to keep the benefit out of rival factions' hands. Otherwise, Perimeter Defense is a smart base facility to build early on, but kind of pricey (in my opinion) for a small, newly-founded base. This Secret Project is a decent buy, but not on a par with more cost-effective Projects (e.g. Virtual World or Hunter-Seeker Algorithm).

Secret Project animation narrative:
"As the writhing, teeming mass of Mind Worms swarmed over the outer perimeter, we saw the defenders recoil in horror. 'Stay calm! Use your flame guns!' shouted the commander, but to no avail. It is well known that the Mind Worm Boil uses psychic terror to paralyze its prey, and then carefully implants ravenous larvae in the brains of its still-cosncious victims. Even with the best weapons, only the most disciplined troops can resist this horrific attack."
   -- Lady Deirdre Skye,
   "Our Secret War"