Portus Adurni

Portus Adurni is a 3rd-century Roman Saxon Shore fort at the north end of Portsmouth Harbour, widely regarded as the best-preserved Roman fort north of the Alps. Enclosing 9 acres with flint walls 20 feet high and 10 feet thick, the square fort originally had 20 projecting D-shaped towers, of which 16 survive. Likely built by Carausius c. 285-290 AD to defend against Saxon raiders, the fort was later converted into Portchester Castle following the Norman Conquest, and Professor Barry Cunliffe's excavations (1961-1979) transformed understanding of the site.