The ruins of the mediaeval castle at Pevensey stand in one corner of a Roman fort, on what was once a peninsula surrounded by the sea and salt marshes. The Roman fort, named Anderida, was built in about AD 290. It is one of the largest surviving examples in Britain, with two thirds of its walls and towers still standing to almost their original height. Unlike most Roman forts that were built to a standard rectangular plan, Anderida has an irregular oval layout that follows the shape of the peninsular.