Perimeter Defense (SMAC)

is a base facility in Sid Meier's Alpha Centauri. The Perimeter Defense base facility provides a 100% bonus against non-psionic attacks by land-based units. It requires Doctrine: Loyalty to construct.
==Strategy==
The Human Hive get free Perimeter Defenses, which is a strong argument for playing them. In the early game they have immense value. Before Synthetic Fossil Fuels, the common early wars will be Impact (4) weapons against Synthmetal Armor (2) or Plasma (3) armour. Plasma armour behind a Perimeter Defense is not going to be pushed out by Impact Rovers so easily. In fact, with just the first Impact Rover destroyed attacking your Perimeters, the minerals/ecs lost by your opponent are already nearly equal to what the Perimeter cost you, not to mention the fact that you just saved your base. This tends to dissuade human opponents from even attacking. You're threatening them with losing before they even attack you.
Of course its not that simple, because the threat of one Impact Rover probably will cause the defender to build more than just one Perimeter Defense, since if the attacker sees one base is protected and another is not, which is he likely to go for? And if you have to build 5 or 6 Perimeters to scare off a couple of rovers, the attacker has won a material victory. He's made you spend 250-300 minerals and the build queues of half a dozen bases for his cost of 50-60 minerals and two build queues. So turn this on its head, and you have another use for Perimeter Defenses - making your opponents build them. The game is so balanced that the attacker will almost always have the advantage without Perimeters, and the defender the advantage with Perimeters.
If you have the minerals to spare, The Citizens' Defense Force is one of the more expensive Secret Projects. Most "put a facility in every base" SPs cost the same as (5 x facility), but the CDF at 350 minerals costs (7 x facility) It does have the advantage of putting a free Perimeter Defense in every base though, helping when expanding into dangerous areas and may save you cash compared to the usual cat & mouse game between the attacker and the defender. Compared to the other SPs up for grabs at this level. I'd rather take the Empath Guild, PTS or Virtual World though.
Generally, Perimeter Defenses are nice things to have. They're cheap, cost no upkeep and can save your bases. The one warning I would give is that they should only be built in the bases that will need them, as they do still take up the 5 rows that could otherwise be used to build facilities or units that will then return nutrients, population, energy and minerals back to you and make you more powerful in the long run. Remember, the sooner you build these facilities and units, the better, while a Perimeter Defense is just as good if it was built the turn before the attack or 100 turns before.