Game concept
Castle Holding
The game also links it as Castle, Castles, Castle Holdings.
From the game's encyclopedia
Castles are the most heavily fortified Holding Type and the standard type for Feudal, Clan and Tribal Rulers, as well as Holy Orders. They tend to provide more Levies than Taxes.
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.
Castle Holding is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Castles are the most heavily fortified Holding Type and the standard type for Feudal, Clan and Tribal Rulers, as well as Holy Orders." It is part of Holding Type. The game also links it as Castle, Castles and Castle Holdings. 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.