Castle Hills is an earth and timber ringwork and baileys fortress, which probably dates from the late 11th to the mid 12th century. Thonock was held by Roger of Poitou in 1086, by the Count of Mortain in 1115 and the 'castle of Gainsborough', was granted by King Stephen to William de Roumare, earl of Lincoln by 1146. The site became an important residence in the late 12th and 13th century and remained in use into the 15th century. The manor remained part of the of the Duchy of Lancaster until the mid 16th century, by which time it had been abandoned. Standing against a steep scarp to the west, the roughly circular banked and ditched ringwork is protected by a semi-circular bailey to the north and a large kidney-shaped bailey to the south and east. A mile south is Gainsborough Old Hall and 8 miles south-west is Hayton Castle.