Portchester Castle is a medieval fortress, constructed within the walls of a Roman Saxon Shore Fort (Portus Adurni), located in Portchester, England.
Saxon Shore forts are defensive fortifications, built by the late Roman Empire to defend the coast of the Roman province of Britannia (Britain), and the opposite side of the English Channel.
During the 3rd century, the Roman Empire was weakened by the succession of brief emperors on the throne, internal fighting and invading tribes encroaching the Empires borders and frontiers.
Britannia was no exception, with repeated Saxon and Frank pirates raiding the coastline. To counteract this threat, a great chain of forts called the Saxonicum (Saxon Shore) was commissioned to protect the Roman population and strategically important sites from the raiding parties.