The remains of the Roman fortification, the camp of Legio IV Flavia, are very poorly preserved since they lie under the foundations of younger fortifications or were completely destroyed during later construction. Although the remains of the camp can only be seen in rare places today, their layout testifies to a spacious fortification, the defended area of which was about 20 hectares.

Entrance to the Roman castrum — the so-called Porta Decumana — was located in the axis of today’s Knez Mihailova Street, in Pariska Street, as well as under the building of the Belgrade City Library, at the entrance to Kalemegdan Park. The foundation of the monumental gate, reinforced with two inner and two outer square towers, can be seen today in the Roman Hall of the Belgrade City Library. On the opposite side, below the medieval corner tower of the Upper Town (today’s Despot Tower), there used to be a corner tower of a Roman castrum. This square-based tower, with a side length of about 9 metre long, has only been partially explored. On its northern side, under the foundations of the medieval northwestern rampart, an older Roman rampart about 110 metres long was discovered; it is still seen today in the height of one to two rows of stone. On the other side of the tower, under the escarpment and the half tower of the medieval rampart, was the route of the northeastern rampart of the castrum.