The fort was built to protect Romans and Britons against the Barbarians (Vikings).

Jack Crane

Portus Adurni was a Roman fortress in the Roman province of Britannia. Listed in the Notitia Dignitatum, it is generally accepted as having been located at grid reference SU624045 adjoining Portchester in the English county of Hampshire and was later converted into a medieval castle known as Portchester Castle. It is the best preserved Roman fort north of the Alps.[1]

Portus Adurni

Portus Adurni was a Roman fortress in the Roman province of Britannia. Listed in the Notitia Dignitatum, it is generally accepted as having been located at grid reference SU624045 adjoining Portchester in the English county of Hampshire and was later converted into a medieval castle known as Portchester Castle. It is the best preserved Roman fort north of the Alps.

• http://en.wikipedia.org/wiki/Portus Adurni
• A History of Britain (TV series)