Norwich Castle is a medieval royal fortification in the city of Norwich, in the English county of Norfolk. William the Conqueror (1066–1087) ordered its construction in the aftermath of the Norman Conquest of England. The castle was used as a gaol from 1220 to 1887. In 1894 the Norwich Museum moved to Norwich Castle. The museum and art gallery holds significant objects from the region, especially works of art, archaeological finds and natural history specimens. The historic national importance of the Norwich Castle site was recognised in 1915 with its listing as a scheduled monument. The castle buildings, including the keep, attached gothic style gatehouse and former prison wings, were given Grade I listed building status in 1954. The castle is one of the city's twelve heritage sites. History Norwich Castle was founded by William the Conqueror some time between 1066 and 1075 and originally took the form of a motte and bailey. Early in 1067, William embarked on a campaign to subjugate East Anglia, and according to military historian R. Allen Brown it was probably around this time that the castle was founded. The earliest recorded incident at the castle is in 1075, when it was besieged by troops loyal to William to put down a rebellion known as the Revolt of the Earls, co-led by Ralph de Gael, Earl of Norfolk. Ralph went abroad to try and rally support from the Danes leaving his wife Emma in charge of the garrison. The support failed to materialise and the rebellion was put down. The siege lasted three months and ended when Emma secured promises that she and her garrison would be unharmed and given safe passage out of the country. Norwich is one of 48 castles mentioned in the Domesday Survey of 1086. Estimates suggest that between 17 and 113 houses were destroyed in the building of the castle. Excavations in the late 1970s discovered that the castle bailey was built over a Saxon cemetery. The historian Robert Liddiard remarks that "to glance at the urban landscape of Norwich, Durham or Lincoln is to be forcibly reminded of the impact of the Norman invasion". Until the construction of Orford Castle in the mid-12th century under Henry II, Norwich was the only major royal castle in East Anglia. About the year 1100, the motte was made higher and the surrounding ditch deepened. The stone keep, which stands today, was built