Portus Adurni
Portsmouth, United Kingdom · built 300
Portus Adurni was a Roman fort in the Roman province of Britannia situated at the north end of Portsmouth Harbour. It was part of the Saxon Shore, and is the best-preserved Roman fort north of the Alps. Around an eighth of the fort has been excavated. It was later converted into a medieval castle known as Portchester Castle.
Archaeological site data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).