a special construction, perhaps only for symmetry with the now-destroyed tower of the southeast corner. At any rate, it has a diameter of 1.2 metres, a height of 2.10 metres and its walls are 0.90 metres thick. Northwest side The greater part of the wall of this side, after the aforementioned circular tower is buried under modern houses. Presumably, the porta principalis dextra was located on the location of a courtyard off the Via Don Giovanni Minzoni. The wall then proceeded along the line of the facades of the houses on the south side of the modern Via San Gaspare del Bufalo, passing the sixteenth century Trident of the streets, and ending at the modern piazza San Paolo, where some remains were found during some hydraulic work in 1904, arranged in horizontal layers to deal with the steep slope of the terrain. There the wall meets the corner described above in the section on the northeast side. Road network The road system of a Roman castra was extremely organised and regular and consisted of a basic system of two perpendicular main streets with smaller streets running parallel to them. The main streets were the via praetoria (Headquarters Street) and the via principalis (Parade Ground Street). The former ran the whole length of the castra, connecting the porta praetoria (Headquarters Gate) with the porta decumana (Tenth Gate), passing through the praetorium (Headquarters), while the via principalis crossed the camp in the other direction, connecting the two portae principales (Parade Ground Gates), distinguished as sinistra (Left) and dextra (Right). At Albano, stretches of both of these streets have been excavated - only a short stretch of the via praetoria, near the homonymous gate on the modern Alcide De Gasperi Street, while two stretches of the via principalis survive: one near the porta principalis sinistra and the other on San Francesco d'Assisi Street, which was discovered during the archaeological excavations of 1915–1916, 1.10 metres below the modern ground level. This stretch is very important because so much of crepido (sidewalk) facing the gutter was found. In the 1980s some excavations carried out by the Museo civico of Albano Laziale and the Ramacci company on the site of a demolished seminary on Castro Pretorio Street discovered the intersection between the via principalis and one of roads running parallel to the via praetoria. This street had been blocked with peperino pilasters in the Medieval period