Æthelthryth (also known as Etheldreda) founded an abbey at Ely in 673; the abbey was destroyed in 870 by Danish invaders and was rebuilt by Æthelwold, Bishop of Winchester, in 970. Construction of the cathedral was started in 1083 by a Norman abbot, Simeon. Alan of Walsingham's octagon, built over Ely's navecrossing between 1322 and 1328, is the "greatest individual achievement of architectural genius at Ely Cathedral", according to architectural historian Nikolaus Pevsner. Building continued until the dissolution of the abbey in 1539 during the Reformation. The cathedral was sympathetically restored between 1845 and 1870 by the architect George Gilbert Scott. As the seat of a diocese, Ely has long been considered a city; in 1974, city status was granted by royal charter.
  Ely is built on a 23-square-mile (60 km2) Kimmeridge Clay island which, at 85 feet (26 m), is the highest land in the Fens. Major rivers including the Witham, Welland, Nene and Great Ouse feed into the Fens and, until draining commenced in the 17th century, formed freshwater marshes and meres within which peat was laid down. There are two Sites of Special Scientific Interest in the city: a former Kimmeridge Clay quarry, and one of the United Kingdom's best remaining examples of medieval ridge and furrow agriculture.