Claustra Alpium Iuliarum

[2] Unlike a linear rampart, the Claustra consisted of a series of interconnected fortifications with its center at Castra ad Fluvium Frigidum (in the area of today's town of Ajdovščina, the Vipava Valley); other important fortresses were Ad Pirum on today's Hrušica Plateau and Tarsatica, now a part of the city of Rijeka.

In the year 6 AD the Great Illyrian Revolt took place threatening the Roman heartland.

Subsequently, in order to protect Italy, a series of walls and fortifications were gradually erected around the area of the strategic Postojna Gate.

[2] Although this development was done subsequent to a major invasion of Northern Italy by the Alemanni in 271, Whittaker indicates that inner fortification lines were primarily aimed to secure the internal stability of the empire rather than keeping barbarians out.

In this battle the eastern emperor Theodosius I prevailed over his western rival Eugenius and by his victory secured Christianity as the main religion of the empire.