Garrison
Many structures can be garrisoned by invisible "AI soldiers", allowing them to automatically fire at enemies they see.
Garrisoning Structures
Bases can provide garrison to the structures around them in a radius that depends on the base type. However they only provide garrison when they've unlocked the Small Garrison 
upgrade or when they have the Provisional Garrison 
upgrade unlocked and 10+ players have set their spawn at that base.
You can tell if a structure is garrisoned or not by the appearance of its flag, which flies proudly if it is garrisoned but is small and tattered if it is not. For defensive buildings with windows (Town Base, Garrisoned House, Safe House), the AI is represented by the size of the cloth under the window.
It's common to build bases in certain locations specifically to provide garrisons to defenses.
Power is not required, it only helps the AI's range at night.
AI Vision
When placing the blueprint of a garrisonable structure its firing arc and range are visible. Both the range and arc depends on the structure type, a Rifle Pillbox can see 360 degrees while a Machine Gun Pillbox has a narrow cone of vision but longer range. Within that range the AI can only fire at targets it can actually see, it can't shoot targets behind solid structures.
At night the AI range decreases and becomes identical to the circle of light that players have. It reaches its minimum of 14m at midnight. Soldier, vehicles, and structures can be closer to garrisoned defenses without being shot at night than during the day. However, Bunker defenses with powered spotlights (Rifle Garrison, AT Gun Garrison, etc...) keep their daytime range during the night. Only the Howitzer Garrisons and Coastal Guns always keep their day-time range.
Unlike players, AI-controlled defenses are unfazed by bushes and will shoot at you even if you are hidden in them. The AI is also not blinded by smoke but it reduces its maximum range, fire rate, accuracy, and time to acquire a target.
Certain garrisoned defenses have limitations on what they can shoot: Anti-Tank Pillbox and AT Gun Garrison can't shoot infantry while Coastal Guns can only shoot vehicles on water (except Large Ships).
Retaliation
Pillboxes and Bunker Garrisons retaliate when fired upon, their range increases significantly and they keep firing for a set duration that depends on the structure type (15s for Howitzer and AT Gun Garrisons, between 5 and 7s for the others). Retaliation fire ignores night limitations, the AI will shoot at what attacked it even if it was outside its night range. Town Bases, Garrisoned Houses, and Safe Houses do not have this retaliatory behavior. Howitzer Garrisons retaliate only when hit by High Explosive damage.
For Bunker Garrisons that are part of a bunker complex the retaliation also occurs if any bunker piece that is part of the same bunker complex is hit (trench connection doesn't count). Thus putting different types of bunker garrison in the same block can counter multiple types of attack (infantry, vehicle, artillery). However unlike a solo garrison, they will only retaliate if the hit decreases their bunker complex's health (e.g. flamethrowers and small arms won't trigger them).
If the shooter was in a vehicle/structure and dismounts the retalation fire keeps targeting their old seat in the vehicle/structure and not the shooter now on foot.
Suppression
Pillboxes, Rifle Garrisons, and Machine Gun Garrisons can be suppressed by firing at them, temporarily preventing them from shooting back. See AI Defense Suppression.
Losing Garrison
If a structure is garrisoned when all the garrison providers in range of it are destroyed then it will stay garrisoned for ten minutes, after which it loses its garrison and will not fire on enemies until the garrison is restored.
Garrisonable Structures
- AT Gun Garrison
- Anti-Tank Pillbox
- Coastal Gun
- Garrisoned House
- Howitzer Garrison
- Machine Gun Garrison
- Machine Gun Pillbox
- Relic Base
- Rifle Garrison
- Rifle Pillbox
- Safe House
- Seaport
- Town Base
Entering Garrisoned Defenses
Players can get inside most garrisoned defenses but they can't fire the structure's weapon, only the AI soldiers can. However the players can shoot their own equipped weapon from inside. Being inside the structure renders the player untouchable but they can be killed by Gas Grenades, just like when they are in a vehicle.
The only garrisoned structures that can't be entered by players are: AT Gun Garrison, Howitzer Garrison, Coastal Gun, Seaport.