Castra ad Fluvium Frigidum


Castra ad Fluvium Frigidum, also simply Castra, referred to as mutatio Castra in Itinerarium Burdigalense, was a Late-Roman fortress 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.
The fortress was built on the grounds of an Early Roman settlement next to the confluence of the Hubelj River and Lokavšček Creek in the Vipava Valley, along the road Via Gemina from Aquileia to Emona, in the early 270s. It had a permanent military crew and command. In ancient sources, it is related to the Battle of Frigidus, between the army of the Eastern Emperor Theodosius I and the army of Western Roman ruler Eugenius, in 394, albeit it had a peripheral role, as it was probably used only for the encampment of Eugenius's infantry. It was depicted in the 5th-century register Notitia Dignitatum. It was demolished by Attila, the ruler of the Huns, in 451.
The fortress in a form of an irregular polygon had a defensive wall and fourteen defensive towers. Its width was c. , its length was c. , and its full perimeter was c. . The wall had a thickness of. The square towers had a height of at least. Seven towers and a part of the wall have been preserved. A ditch surrounded the fortress. Extensive archaeological research has been conducted at the site. In addition to the already known ruins, several urn and skeletal graves, smaller Roman baths with a caldarium, and, in September 2016, the eleventh defensive tower have been discovered.