Portus Adurni

Portus Adurni stands at the northern edge of Portsmouth Harbour, a Roman fort built in the 3rd century as part of the Saxon Shore defences. It occupies a commanding square position covering nine acres, with walls twenty feet high and ten feet thick. The fort’s outer circuit remains largely intact, with only four bastions lost over time. Its towers and inward-indented gates show careful planning for defence against seaborne raids. Historians link its construction to the Carausian Revolt, when the rebel commander minted coins found on site. The fort’s survival makes it the best-preserved Roman fort north of the Alps. Evidence suggests it was built during Carausius’s revolt, when instability in Gaul led to pirate attacks across the Channel. After Rome regained control, the site became an Anglo-Saxon high-status residence with a great hall and tower. Later it served as a Norman castle and medieval palace, then a prison during the Napoleonic Wars. Its sixteen centuries of continuous use explain why the walls still stand so well. Visitors walk along the flint-and-limestone walls, noticing the varied textures from different gangs of builders. The inward gates trap any attacker between high walls on three sides, a design used from Augustus’s time to the empire’s fall. Inside, the medieval castle rises within the Roman enclosure, its gateways rebuilt by later rulers. The outer bailey wall follows the same ancient line, enclosing a space that has housed halls, towers and gaols. From the walls, views stretch over Portsmouth Harbour and the surrounding land.