The 4th century walls of this Roman Saxon Shore fort are still standing to their original height
overlooking Portsmouth Harbour. The site is square with each wall being 600 foot in length with the keep and bailey walls, protected by a moat, in the north west corner. The impressive keep was built in 1120, the height being raised by two storeys to ninety foot high later in the century, when the Royal Palace buildings were also constructed. At the end of the 14th
century they were rebuilt and these are the visible remains. It was used as a prison from the 13th century until the end of the 18th century. A priory was built in one corner in 1133 and a church still stands inside the walls.