Motte and Bailey
From Conservapedia
A Motte and Bailey is a form of medieval castle. They were common around the 11th century AD especially in England and Ireland. Eventually they became obsolete and were replaced with bigger, stronger castles. The Motte was a natural or manmade hill 10 to 100 feet high with a castle or keep on top. The bailey was an enclosed area surrounded by a fence. Usually the structures were made of wood.
See also
Links
List of Motte and Bailey Castles