Game concept
Garrison
From the game's encyclopedia
The Garrison of a Fortified Holding protects it from being Besieged by smaller Armies.
The besieging Army needs to have more Soldiers than the Garrison in order for the Siege to progress.
When a Siege is lost, the Garrison is set to 25. From there, the Garrison refills every month depending on the Garrison Reinforcement Rate until it reaches its max capacity.
When a Siege begins, the manning of the Garrison affects the required amount of Siege Progress.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Garrison is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The Garrison of a Fortified Holding protects it from being Besieged by smaller Armies." Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.