Game concept
Fortification Building
The game also links it as Fortification Buildings.
From the game's encyclopedia
Fortification Buildings are Buildings whose primary function is to either increase their holding's Fort Level or provide bonus Advantage for defenders.
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.
Fortification Building is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Fortification Buildings are Buildings whose primary function is to either increase their holding's Fort Level or provide bonus Advantage for defenders." It is part of Building. The game also links it as Fortification Buildings. 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.