The straightforward answer is: nobody knows...
This Q&A first appeared in the Christmas 2012 issue of BBC History Magazine
Friday 16th December 2016
The Normans, as is widely appreciated, were originally Norsemen: Vikings who settled in the area around the Seine estuary in the late ninth and early 10th centuries. The traditional date for the founding of Normandy is AD 911, when the authority of the Norman leader, Rollo, was recognised by the king of France.
Castles appeared somewhat later, with the earliest examples being constructed around the turn of the first millennium. They differed from earlier fortifications by being smaller and taller: the distinctive feature of early castle design was the great artificial mound of earth, or motte. Dating a mound of earth is difficult, since it relies on the discovery of datable ‘small finds’, and so establishing a precise chronology for mottes is impossible. Nor is it possible to say for certain how and why the design originated, other than to observe that the rise of castles seems to coincide with an intensification of lordship across northern France in the decades around the millennium. Evidently someone had the notion of building a great mound of earth to assert his power and the idea caught on fast.
The Normans began ditching their Norse culture and adopting French customs almost from the minute of their arrival. During the 10th century, for example, they embraced Christianity, the French language and the habit of fighting on horseback. Learning how to build castles was therefore simply part of an ongoing process of acculturation. According to contemporary chroniclers, a great surge of castle-building took place during the troubled years of William the Conqueror’s boyhood in the 1030s and 1040s.
“Lots of Normans, forgetful of their loyalties, built earthworks in many places,” wrote William of Jumièges, “and erected fortified strongholds for their own purposes.”
Answered by Marc Morris, historian, author and broadcaster.