Fortify your Core
Units come and go; defenses stay. The wall and the emplacements are built at a Core and fight for it every time it is attacked, whether or not anyone is home. Every class builds the same ones: a class changes only what they cost and how fast they go up.
The wall
One per Core, raised a level at a time, and needing no research. A wall makes every unit defending the Core harder to kill, and it lends the same strength to the defense of the Colony's Holdings. It never helps an attacker, and it cannot be built on a landmark.
An upgrade keeps the current level working until the new one is finished.
Emplacements
Emplacements are fixed weapons built at a Core's Workshop. They fight on their own, even with no garrison, and eat no food. Like units they come in light, medium and heavy pairs, with the Fortress above them, and each weight class opens through its own line of fortification research.
Loading the roster.
Guarding the Holdings
The Core's garrison never leaves to meet a raid. A raided Holding is defended instead by a projection of the Core's emplacements: a share of them, at full strength, with the wall and the Armory behind them. Outworks widens that share until the whole ring answers every raid. Any army standing on the Holding fights too.
After the fight
Each emplacement a fight knocks out is either rebuilt or destroyed once the fight ends, and heavier kinds come back more often. The ones that stood are untouched. An emplacement is never left damaged, so a Core is always exactly as strong as the weapons standing on it.
A raid on a Holding fights only a share of the ring, so only that share can fall to it, but several raids on one Colony in the same turn each meet their own full share, and their losses add up. Capture wipes every emplacement and knocks the wall down a level. The Armory makes everything defending the Colony hit harder and last longer.
Fortifying a landmark
A landmark you hold can carry emplacements too, ordered through your capital's Workshop and paid from its stockpile. No wall stands on a landmark, and everything built there is destroyed the moment you stop holding it, however that happens.