Castra ad Fluvium Frigidum

Castra ad Fluvium Frigidum (Latin for 'Fortress by the Cold River'), also simply Castra (Slovene: Kastra), referred to as mutatio Castra (Castra relay station) 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 (now Carinthia, Austria) to the Učka mountain range (now Croatia). On its grounds, the Late Mediaeval 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 (now in southwestern Slovenia), along the road 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 encampent 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.

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