ETYM AS. castel, from Latin castellum, dim. of castrum a fortified place, castle.
A large building formerly occupied by a ruler and fortified against attack.
Fortified building or group of buildings, characteristic of medieval Europe. The castle underwent many changes, its size, design, and construction being largely determined by changes in siege tactics and the development of artillery. Outstanding examples are the 12th-century Krak des Chevaliers, Syria (built by crusaders); 13th-century Caernarvon Castle, Wales; and 15th-century Manzanares el Real, Spain.
Structure.
The main parts of a typical castle are the keep, a large central tower containing store rooms, soldiers’ quarters, and a hall for the lord and his family; the inner bailey or walled courtyard surrounding the keep; the outer bailey or second courtyard, separated from the inner bailey by a wall; crenellated embattlements through which missiles were discharged against an attacking enemy; rectangular or round towers projecting from the walls; the portcullis, a heavy grating which could be let down to close the main gate; and the drawbridge crossing the ditch or moat surrounding the castle. Sometimes a tower called a barbican was constructed over a gateway as an additional defensive measure.
Early castles (11th century) consisted of an earthen hill (motte) surrounded by wooden palisades enclosing a courtyard (bailey).
The motte supported a wooden keep. Later developments substituted stone for wood and utilized more elaborate defensive architectural detail. After introduction of gunpowder in the 14th century, castles became less defensible and increases in civil order led to their replacement by unfortified manor houses by the 16th century. Large stone fortifications became popular again in the 18th century, particularly those modeled after the principles of fortification introduced by the French architect Vauban, and were built as late as the first half of the 19th century. In the late 19th century, castlelike buildings were built as residences for the wealthy as part of the Romantic revival in Europe and America.

1. Interchanging the positions of the king and a rook; SYN. castling.
2. The chessman that can move any number of unoccupied squares in a direction parallel to the sides of the chessboard; SYN. rook.

To move the king two squares toward a rook and in the same move the rook to the square next past the king; in chess.