(Latin for 'Barrier of the Julian Alps'; hereby, the term Julian Alps refers to the wider mountainous and hilly region from the Julian Alps to the Kvarner Gulf) was a defense system within the Roman Empire between Italia and Pannonia that protected Italy from possible invasions from the East. 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. 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. They had been governed from the town of Aquileia.