I've checked the following files:

data\descr_building_battle.txt & descr_building_battle_enums.txt (neither mention these buildings and they're not faction specific)

data\export_descr_buildings.txt (my faction is listed under all the 'building convert_to_castle' and 'building convert_to_city' buildings)

data\export_descr_buildings_enums.txt (my faction's culture is listed here)

data\text\building_battle.txt (does not mention these buildings and is not faction specific).

data\text\export_buildings.txt (my faction's culture is listed here)

Are there any other files concerning buildings.

Also the error mesage itself is odd since a village, not a town is converted to a wooden castle; though a large town does convert to a castle.