The Old Gate or the Roman Arch is not, as was assumed earlier, a Roman triumphal arch, but a monumental main entrance into the heart of the military headquarters of the late Roman empire Tarsatica, an ancient town on whose ruins medieval Rijeka rose. The people of Rijeka were right to trust their instincts and call it simply The Gate; the term Roman Arch became gradually more accepted from the time of Classicism, when the documentation and the research of this monument began. The profiled facade of the Arch has been preserved only in fragments, the other parts fell off long time ago or have been taken away to be incorporated into the houses of medieval Rijeka. The Tarsatica Principia was the main camp, the supply base and the starting point of the Claustra Alpia Iuliarum, dozens of kilometres long intermittent defence walls, towers, guard stations, and larger fortifications positioned in key communication lines and elevated points, with the aim of stopping barbaric invasions towards Italy, the heart of the Roman Empire. From the perspective of logistics, it made sense to choose the centre of the ancient port of Tarsatica, a predecessor of the medieval and modern Rijeka, as the location for a kind of background garrison of this system. Only a small portion of the remains of the defence system has been researched, and even smaller portions are on display in Kalvarija in the area of Kozala in Rijeka, near Jelenje and Studena in the hinterland of Rijeka, and near the village of Prezid in Gorski Kotar, and they stretch further into the region of modern Slovenia, and a smaller part extends as far as Italy and Austria.