Ruins of the late-antiquity castrum

The back of the church coincides with a side of a rectangle, whose sides – each some 30 metres long – enclose a significant area of the Old Town. In the late antiquity this area was occupied by a late antiquity castrum, the so-called Praetorium of the Alpine enclosure, a military command of the troops in charge of surveillance, maintenance and defence of the system of fortifications that extended from Rijeka, i.e. the antique Tarsatica, to the north and protected the inside lands of the Roman Empire. The masonry of the Praetorium has been preserved to a certain height in the derelict houses that were built upon it in the subsequent centuries, following its closed structure. It is easy to depict owing to the alternate strata of roughly carved stone and the levelling Roman bricks. From the outside this opus mixtum is best seen right below the rear of St. Sebastian's and the north front of the buildings erected above the Praetorium itself.