Claustra Alpium Iuliarum

The Claustra Alpium Iuliarum highlighted on a map of the Roman Empire
Location of Claustra Alpium Iuliarum outposts with modern national borders of Slovenia and neighbouring countries.

Claustra Alpium Iuliarum (Latin: "Barrier of the Julian Alps") was a defense system within the Roman Empire between Italia and Pannonia that protected Italy from possible invasions from the East.[1] It secured the Postojna Gate, the land link between the eastern and western part of the empire, and thus the Claustra represented an inner border defense of the empire.[2] Contrary to a linear limes the Claustra consisted of a series of interconnected fortifications with its center at Ad Pirum at today's Hrušica.


In the year 6 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.[1] Most of the construction was done after 284 under Diocletian and Constantine I.[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.[2] The fortification system included Forum Iulii (in the area of today's Cividale del Friuli), Tarsatica (in the area of today's Rijeka) and followed the valley of the Idrijca river. It stretched over the Postojna Gate to the hills south of Emona (in the area of today's Ljubljana). In-depth fortifications along the Roman road started at the fortress of Castra ad Fluvio Frigido (the remains of which are today still visible in Ajdovščina), which was the centre of the system,[3] and ended at Nauportus (in the area of today's Vrhnika). The hill fortress of Ad Pirum was typically manned with 500 soldiers but could keep up to 100,000 soldiers. Ad Pirum’s walls were unearthed by Austrian and Italian archeologists and shown to be at a height of 8 m and a thickness of 2 m; the wall towers were 10 m high.


Castra Alpium Iuliarum saw a number of battles. Early fortifications may have been useful in 169 when the Marcomanni attempted to enter Italy but proved inadequate when the Alemanni invaded Italy in 271. In 351 Constantius II took Ad Pirum during his fight against his challenger Magnentius. Most importantly, the Battle of the Frigidus took place in 394 between Castra and Ad Pirum. 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.

After the 5th century the Roman fortifications fell into disrepair. Today selected sections have been restored by archeologists.


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  1. 1 2 Potocnik AJ. "Claustra Alpium Iuliarum". Retrieved July 21, 2010.
  2. 1 2 3 C.R.Whittaker. Frontiers of the Roman empire. A social and economic study. Baltimora & London, 1997. p. 197.
  3. Pavlin, Primož. "Poznoantična utrdba Kastra (Ajdovščina)" [The Late-Antique Fortification of Castra (Ajdovščina)]. DEDI - enciklopedija naravne in kulturne dediščine na Slovenskem (in Slovenian). Retrieved 8 April 2014.

Coordinates: 45°51′49″N 14°6′42″E / 45.86361°N 14.11167°E / 45.86361; 14.11167

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