This is the oldest part of the great wall, largely built by the Qin dynasty. The sands of time might have worn down stone, brick, and mortar, but it's still a formidable piece of fortification.

Source: game/common/buildings/tgp_great_project_buildings.txt, game/common/holdings/00_holdings.txt, game/common/script_values/00_building_values.txt and the English localisation, Crusader Kings III 1.20.0.3.