add_random_fortification_building_effect

add_random_fortification_building_effect = {
	if = {
		limit = { free_building_slots > 0 }
		save_scope_as = building_scope
		custom_tooltip = random_fortification_building_tt
		hidden_effect = {
			random_list = {
				10 = { # Ramparts
					trigger = {
						NOT = { has_building_or_higher = ramparts_01 }
						building_ramparts_requirement_terrain = yes
						building_requirement_castle_city_church = { LEVEL = 01 }
					}
					add_building = ramparts_01
				}
				10 = { # Curtain Walls
					trigger = {
						NOT = { has_building_or_higher = curtain_walls_01 }
						building_curtain_walls_requirement_terrain = yes
						building_requirement_castle_city_church = { LEVEL = 01 }
					}
					add_building = curtain_walls_01
				}
				10 = { # Watchtowers
					trigger = {
						NOT = { has_building_or_higher = watchtowers_01 }
						building_watchtowers_requirement_terrain = yes
						building_requirement_castle_city_church = { LEVEL = 01 }
					}
					add_building = watchtowers_01
				}
				10 = { # Hill Forts
					trigger = {
						NOT = { has_building_or_higher = hill_forts_01 }
						building_hill_forts_requirement_terrain = yes
						building_requirement_castle_city_church = { LEVEL = 01 }
					}
					add_building = hill_forts_01
				}
			}
		}
	}
}

As written in game/common/scripted_effects/00_building_effects.txt.