Portus Adurni was a Roman fort in the Roman province of Britannia situated at the north end of Portsmouth Harbour. It was part of the Saxon Shore, and is the best-preserved Roman fort north of the Alps. Around an eighth of the fort has been excavated. It was later converted into a medieval castle known as Portchester Castle. Identification The name Portus Adurni appears only in the list of Saxon Shore forts in the 5th-century Notitia Dignitatum, and the name is usually identified with Portchester, although it has occasionally been identified with the Roman fort at Walton Castle, Suffolk (which has now been washed away by the sea). Portus Adurni may be identical with the Ardaoneon listed in the Ravenna Cosmography, and Rivet and Smith derive both names from the British "ardu-" meaning "height". This derivation fits Portchester (which lies beneath Portsdown Hill) better than a flat location such as Walton Castle. Roman fort The fort was built during the 3rd century as part of the so-called Saxon Shore forts to protect the southern coastline of Britain, possibly from Saxon raiders. It occupies a commanding position at the head of Portsmouth Harbour. The fort is square, enclosing an area of 9 acres (36,000 m²) with outer walls 20 feet (6 m) high, 10 feet (3 m) thick, 210 yards (200 m) long and constructed of coursed flint bonded with limestone slabs. Square-shaped forts became widely used during the 3rd century, being highly practical and defensible. Portus Adurni has towers all along the walls, as well as towers that angle out at the corners. Historians feel this paramount concern for defensibility may reflect the seriousness of the Saxon raids during this time, or the defensive pressures of the Carausian Revolt (see below). The gates of Portus Adurni are of particular interest: they are indented inwards, so as to trap the enemy in an area exposed to walls on three sides; this technique became widely used from the Augustinian age to the fall of the Empire. Unusually for a building of this period, most of the walls and bastions are complete. It has lost only four of its bastions. The walls themselves have been quarried to provide stone for later additions, but from the outside at least they appear much as they did when they were first erected. The walls were built in sections, by groups of gangs, which explains the different