medieval European stronghold, generally the residence of the king or lord of the territory in which it stands. The word is sometimes applied to prehistoric earthworks, such as Maiden Castle, England; and the word is also applied, in various linguistic forms (e.g., château, castello, or Burg), to princely mansions or country seats.

The castle developed rapidly in western Europe from the 9th century. Fortifications built in France in the 10th century often included a high mound encircled by a ditch and surmounted by the leader's particular stronghold, as in the castles at Blois and Saumur. Later, one or more baileys or wards (grounds between encircling walls), were enclosed at the foot of the mound. During the 11th century this type of private fortress, known as the “motte [mound] and bailey” castle, spread over western Europe.