The holder of the office of the Bishop of Durham, Bishop William Walcher at the time, was appointed by the King to exercise royal authority on his behalf, with the castle being his seat. According to UNESCO, Walcher "purchased the earldom Northumbria and thus became the first of the Prince-Bishops of Durham, a title that was to remain until the 19th century, and was to give Durham a unique status in England. It was under Walcher that many of the Castle’s first buildings were constructed. As was typical of Norman castles, it consisted of a motte (mound) and an inner and outer bailey (fenced or walled area). Whether the motte and inner bailey were built first is unknown. There is also debate about whether or not Durham Castle was originally a stone or a wooden structure. Historic sources mention that its keep (fortified tower) was built of wood, but there is enough archaeological evidence to indicate that even in the late 11th century when it was first built, it had numerous stone buildings.