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 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 (now in southwestern Slovenia), along the road Via Gemina from Aquileia to Emona, in the early 270s.

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.

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.