 "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.The fort was built during the 3rd century as part of the Saxon Shore forts to protect the southern coastline of Britain, possibly from Saxon raiders.