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Portus Adurni was a Roman fortressmarker in the Roman province of Britannia. Listed in the Notitia Dignitatum, it is generally accepted as having been located at adjoining Portchestermarker in the Englishmarker county of Hampshire and was later converted into a medieval castle known as Portchester Castlemarker. It is considered by many to be one of the finest Roman fortifications remaining in Europe.

Roman fort

The fort was built during the 3rd century to protect the southern coastline of Britain from possibly Saxon raiders (part of the Saxon Shore) and occupies a commanding position at the head of Portsmouth Harbourmarker. 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 third century, being highly practical and defensible. Portus Adurni boasts 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 holding point of 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.

Unusual for a building of this period, the majority of the walls and bastions are complete. It has lost only four of its bastions and, although the walls themselves have been quarried to provide stone for later additions, from the outside at least, they appear much as they did when they were first built. The walls were made in groups, by groups of gangs, which explains the different textures as you walk along the outerperimeter.


The Saxon shore forts, including Portus Adurni, were built during the mid to late 3rd century amid increased instability in northern eastern Gaul and the Rhineland, eventually leading to the Roman evacuation of this area. The forts were intended to maintain control over the region, monitor crucial shipping and trade, as well as defend against raids from across the English Channelmarker. These forts became defensive holding points for the rebel Carausius, who in 285 was given the task of fixing the Saxon piracy problem in the English Channel. Carausius was eventually charged with keeping the pirate's booty for himself, at which point he retreated to Britain and proclaimed himself Emperor. The revolt went on for close to 10 years as other Roman generals tried and failed to dislodge him until he was eventually murdered by one of his aides, Allectus in 293, and Britain was finally taken back by 296. There is still a question as to who built Portus Adurni. While some of the Saxon shore forts were built under previous commanders and emperors, some were built by Carausius during his revolt. Evidence suggests that Portus Adurni was built during the time of Carausius. Of the coinage uncovered at the site, a large portion were minted by Carausius acting as emperor during the time of his revolt.

Anglo-Saxon high-status residence

Even after the departure of the Roman Army Portus Adurni's location and strong walls made it attractive as a fortress. The fort became an Anglo-Saxon high-status residence with great hall and tower. Later the Roman gateways were rebuilt as well.

Later history

The circuit walls of the fort became the outer bailey wall of a Norman castle and later medieval palace. It was used as a gaol during the Napoleonic Wars. Its exceptional condition can therefore be attributed to the fact that, despite short periods of abandonment and longer periods of neglect, the fort was occupied for almost sixteen centuries. The site is owned by the Southwick Estate but managed by English Heritage and is open to visitors throughout the year.


  • Bunson, Matthew. Encyclopedia of the Roman Empire. 1994.
  • Bushe-Fox, J.P.. Some Notes on Roman Coast Defences. The Journal of Roman Studies Vol. 22(1932): 60-72. JSTOR. Northwestern U Lib., Evanston. 6 May 2007
  • Hornblower, Simon, and Antony Spawforth. Carausius, Marcus Aurelius. The Oxford classical dictionary. 3rd ed. Oxford: Oxford University Press, 1996.
  • Hornblower, Simon, and Antony Spawforth. Saxon Shore. The Oxford classical dictionary. 3rd ed. Oxford: Oxford University Press, 1996.
  • Petrikovits, Harold. Fortifications in the North-Western Roman Empire from the Third to the Fifth Centuries A.D. The Journal of Roman Studies Vol. 61(1971): 178-218. JSTOR. Northwestern U Lib., Evanston. 6 May 2007

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