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 disappeared into 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.