Claustra Alpium Iuliarum is a Late Roman barrier system, consisting of several sections of stone walls, towers, forts and fortlets. The time of its creation dates back to the second half of the 3rd century when the Roman Empire was on the verge of collapse and shaken by civil wars and invasions of peoples. The barrier system was part of the military landscape until the beginning of the 5th century. Its purpose was to control the main crossings to ancient Italy (Italia). The positioning of individual barriers was such that it was incorporated as much as possible in the natural formation of the terrain, thus exploiting its defence potential.
In the ancient sources, we find several names for the system. Today, we use the term claustra Alpium Iuliarum. It was first used by the Roman historian Ammianus Marcellinus in the 4th century, who wrote:
“Veritis defensoribus ne captus ab hoste veniret et subornatus, atque contigeret aliquid in civitatis perniciem, quale per Acacium acciderat comitem, quo per fraudem a Magnentiacis militibus capto, claustra patefacta sunt Alpium Iuliarum.” – “The defenders feared that he came after being captured by the enemies who got him on their side and that some sort of danger is threatening the city. The comes Acacius did something similar; he was caught by Magnentius’ soldiers by means of a ruse and so the barriers in the Julian Alps were opened.”
Claustra Alpium Iuliarum is probably the largest architectural venture from the Roman era in the territory of present-day Slovenia and is thus comparable with the largest antique monuments in Europe (for example, the limes near the Danube River and Hadrian’s Wall in England). We can follow it from Rijeka in Croatia (ancient Tarsatica) through western Slovenia to the Julian Alps. More precisely, it runs from Rijeka through Jelenje and continues on the Babno Field (Babno polje), the Bloke Plateau (Bloška planota), Rakitna and Pokojišče. The system then runs above Vrhnika, Logatec, Lanišče and Hrušica and ends at Zarakovec near Cerkno. It thus extends across Slovenia and Croatia; we find it in twelve Slovenian and seven Croatian municipalities. Together with the interruptions, it is about 130 kilometres long. Today, we know more than 30 kilometres of archaeological remains of the barrier walls with more than 100 towers.
In the Late Roman times, civil wars and invasions of foreign peoples on the Roman territory were increasingly frequent and the fighting kept taking place closer and closer to ancient Italy and the heart of the empire, Rome. In the critical times of the second half of the 3rd century, on the order of the ancient strategists and military authorities, the Roman army began to build a barrier line. The area where the barrier system runs was called the Julian Alps (Alpes Iuliae) by the ancient geographers. Despite its hilliness and diversity, it was the easiest route to the Apennine Peninsula. The main task of the barrier system was therefore to control and protect all the areas that could be relatively easily crossed, such as mountain passes, natural passages and roads.
Individual barriers measure from a few hundred meters to a few kilometres in length and the walls are from one to two meters wide. A fort and 4 fortlets are also included in the system. The claustra Alpium Iuliarum system was used for less than a century and a half, after which it was abandoned due to military-political changes.