“Having now established a secure perimeter, we have made ourselves relatively safe from enemy incursions. But against the seemingly random attacks by Planet’s native life only our array of warning sensors can help us, for the Mind Worms infiltrate through every crevice and chew through anything softer than plasmasteel.”
— Lady Deirdre Skye, “The Early Years”
Perimeter Defenses require Doctrine: Loyalty to build. They are cheap in minerals and in energy maintenance and grant a large defensive bonus to any units stationed in the base. Notably, each Hive base starts with Perimeter Defenses active, to reflect how the Hive builds each of their bases underground as a maze of twisting, easily defensible tunnels.
Because of that, one might have expected the quote to come from Chairman Yang. But, instead, we are treated to a quote from Lady Deidre. She dwells on the fact that fortifications do not avail human defenders against mind worms. Because mind worms are terrifying. Of course, that’s not how she puts it. She practically worships the mind worms, even as she also seeks to understand them scientifically. But anything that can burrow straight through concrete as easily as dirt is not to be trifled with.
From this quote, one can start to picture the Gaians early game strategy in the canon. They established a broad sensor net to get early warning of encroaching worms. Then, where the Morganites would passively remain in their bases, relying on the sensor bonus and hypnotic trances to fight off the worms, the Gaians would strike out and attempt to co-opt and redirect the worms away from their settlements. This is the natural extension of their religious injunction to work with the Planet rather than against it to the military realm.
As modeled by the game, the Gaians’ strategy is probably superior most of the time. That’s due to a few factors. First, the psi attacker gets a significant bonus on land. Second, when winning an attack against a stack of mind worms all occupying the same square, a successful attack kills them all. Whereas if you hide behind your static defenses, each mind worm gets to attack. This means that fewer mobile base defenders can protect more bases, as it is rare that lots of worms come on the exact same turn from different directions.
Finally, when defeating a mind worm boil on the attack, the attacking force gets a small amount of energy credits depending on the size of the boil. In the early game, these credits are a very valuable reward for exploration. They’re almost as valuable, in and of themselves, as salvaging the scattered supply pods from the crashed Unity. Later in the game, this potential source of income isn’t a big deal to a faction like the Morganites. But to the Gaians (who can’t run Free Market) or the chronically energy-poor Hive (who don’t really get any benefit from Free Market with their native Economy penalty), the so-called “farming” of mind worms by exploring fungus squares can be a significant boost.