A motte and bailey castle consists of a man-made hill (the motte) supporting a tower and a walled yard (the bailey). (...) Early castle builders looked for a natural hill on which to erect a timber tower, but since a hill might not be available where fortifications were needed, they raised a flat-topped, conical earthen mound by digging a circular trench or ditch the desired diameter and heaping up the dirt in the center.

Ch. 1 : The Great Tower : Norman and Early Plantagenet Castles - Medieval castles (2005)