Motte-and-bailey castle

A motte-and-bailey castle is a European fortification with a wooden or stone keep situated on a raised area of ground called a motte, accompanied by a walled courtyard, or bailey, surrounded by a protective ditch and palisade. Relatively easy to build with unskilled labour, but still militarily formidable, these castles were built across northern Europe from the 10th century onwards, spreading from Normandy and Anjou in France, into the Holy Roman Empire in the 11th century. The Normans introduced the design into England and Wales. Motte-and-bailey castles were adopted in Scotland, Ireland, the Low Countries and Denmark in the 12th and 13th centuries. Windsor Castle, in England, is an example of a motte-and-bailey castle. By the end of the 13th century, the design was largely superseded by alternative forms of fortification, but the earthworks remain a prominent feature in many countries.

Source: Wikipedia