Kvarner Gulf
In the late 3rd century and the early 4th century a fortification system, "Claustra Alpium Iuliarum", which run from the Kvarner Gulf to Cividale, was built by the Roman Empire. Its centre was the fortress of "Castra" or "Castrum ad Fluvio Frigido", the remains of which are today still visible in Ajdovščina.