The barrier walls of the claustra Alpium Iuliarum system extended from the ancient Rijeka (Tarsatica) in Croatia on a steep slope towards the north above the Rječina River canyon. In that time, a building of the military command, called the Principia, was also built in the centre of the fortified city. The city was thus included in the barrier system and it guarded one of the main Roman arterial roads from the Balkan Peninsula, the Pannonian Plain and northern Europe to ancient Italy.
The barrier in Rijeka, which begins on the coast of the Adriatic Sea and with its sections continues towards the north, consists of six sections – the fortified city of Tarsatica, the Calvary Hill (Kalvarija) barrier walls 1–3, the street called Ulica Oktavijana Valića and the Saint Catherine’s Hill (Sveta Katarina). Individual sections are separated from each other and extend in a seeming line from Tarsatica to the Calvary Hill and through a mountain pass to Saint Catherine’s Hill, where they also end. The total length of the barrier walls is 403 m and the fortified city extends over 1,367 m. There are no reliably proven towers on the barrier.
The fortified city of Rijeka stands on a smaller plateau along the otherwise steep coast. The wall was fortified during a political-military crisis in the 3rd century and a permanent military crew was stationed in the city. Approximately during that time, a complex in the city centre was also built or restored, interpreted by experts as the building of the military command of the city, e.g. the Principia. Based on the fine and rough ceramics discovered there, we know that the Principia was in use from the second half of the 3rd century to the time of transition from the 4th to the 5th century, when the building burnt down. Individual military finds, such as parts of military and horse equipment, point to the military character of the building.
In the framework of the CAI barrier system, Tarsatica represents a unique example of an ancient city incorporated directly into a linear barrier line. The Calvary Hill and the Saint Catherine’s Hill offer a good view to the north of the Rječina River canyon and of the neighbouring barriers Jelenje on the Grobnik Field (Grobničko polje) and Studena and towards the east also of Trsat, Solin and the fortified city of Tarsatica. The view reaches the greater part of Kvarner, the islands of Krk and Cres and the coast towards Senia (today’s Senj).