Motte-and-bailey

A motte-and-bailey is a kind of castle, or fortification. Many of them were built in the 11th and 12th century around Europe.
These structures consist of a hill (the motte) and a courtyard (the bailey). The motte was often artificial and on top, a wooden or stone tower was built. The bailey was surrounded by a protective ditch and wall of wooden fence (a palisade). They were relatively easy to build compared to stone buildings. All the same, they were significant and difficult to overcome.
If they were built of wood, this meant they could be burnt down. Also wood rots when it gets wet so the structure needed regular repair. As time went by castles were made of stone, which is much stronger. Palisades were replaced by stone walls, and wooden tower were replaced in stone. 
Motte and bailey castles appeared in England after the Norman Conquest of 1066. They were a common feature in England by the death of William the Conqueror in 1087. Their construction was the start of what was to become a massive castle building programme in England and Wales.