Castle Knob,[1] also known as Castle Gresley castle[2] is a motte and bailey castle in Derbyshire, England. The date of construction is not known but may date to the mid-12th century Anarchy era. The site was long under the ownership of the de Gresley family. The castle survives as earthworks including a 4 metres (13 ft) high motte and three baileys, the largest measuring 70 metres (230 ft) across. The site had been abandoned by the 16th century and, in the mid-20th century, the Royal Observer Corps (ROC) installed a nuclear warfare monitoring post in the central bailey. The site (excluding the ROC post) became a scheduled monument in 1966.