Castra ad Fluvium Frigidum, also simply Castra, referred to as mutatio Castra in Itinerarium Burdigalense, was a Late-Roman fortress (castrum) which constituted the centre of Claustra Alpium Iuliarum, an Ancient Roman defensive system of walls and towers stretching from the Gail Valley to the Učka mountain range. On its grounds, the Late Medieval market settlement of Ajdovščina developed.