 It is the best-preserved Roman fort north of the Alps.Around an eighth of the fort has been excavated.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).