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The fort was built during the 3rd century AD to protect the southern coastline of Britain from Saxon raiders and occupies a commanding position at the head of Portsmouth harbour. It is square with outer walls 20 feet high and 210 yards long and unusually for a building of this period, the walls and bastions are complete. This is probably due to its continued use from the departure of the Roman Army right through to the 19th Century then it was used as a gaol for prisoners of the Napoleonic Wars.