castle

¹ American  

[kas-uhl, kah-suhl] / ˈkæs əl, ˈkɑ səl /

noun

1. a fortified, usually walled residence, as of a prince or noble in feudal times.

   Synonyms:

   citadel, fortress

2. the chief and strongest part of the fortifications of a medieval city.

3. a strongly fortified, permanently garrisoned stronghold.

4. a large and stately residence, especially one, with high walls and towers, that imitates the form of a medieval castle.

   Synonyms:

   château, palace

5. any place providing security and privacy.

   It may be small, but my home is my castle.

6. Chess. the rook.

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verb (used with object)

castled, castling

1. to place or enclose in or as in a castle.

2. Chess. to move (the king) in castling.

verb (used without object)

Chess.

castled, castling

1. to move the king two squares horizontally and bring the appropriate rook to the square the king has passed over.

2. (of the king) to be moved in this manner.

Castle ² American  

[kas-uhl, kah-suhl] / ˈkæs əl, ˈkɑ səl /

noun

1. Irene (Foote), 1893–1969, born in the U.S., and her husband and partner Vernon (Vernon Castle Blythe ), 1887–1918, born in England, U.S. ballroom dancers.

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castle British  

/ ˈkɑːsəl /

noun

1. a fortified building or set of buildings, usually permanently garrisoned, as in medieval Europe

2. any fortified place or structure

3. a large magnificent house, esp when the present or former home of a nobleman or prince

4. the citadel and strongest part of the fortifications of a medieval town

5. chess another name for rook 2

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verb

1. chess to move (the king) two squares laterally on the first rank and place the nearest rook on the square passed over by the king, either towards the king's side ( castling short ) or the queen's side ( castling long )

Other Word Forms

• castlelike adjective
• uncastled adjective

Etymology

Origin of castle

before 1000; Middle English, Old English castel < Latin castellum castellum

Explanation

A castle is a huge, grand home where a king or queen might live. Almost all castles are also fortified against attacks by enemy armies. Most castles were built in the Middle Ages by royalty or other nobility. You can still visit many historic castles in Europe, most of them built of stone and including details like towers and guardhouses. You can also call the chess piece known as a "rook" a castle, for its castle-like shape. The word has an Old English root, castel, or "village."

Example Sentences

The AI castle on a hill boasts best performance/watt training and inference.

From Barron's • Apr. 17, 2026

A cave hidden beneath an 11th Century castle in Pembrokeshire is a "truly remarkable site" which could rewrite Britain's prehistory, researchers say.

From BBC • Apr. 15, 2026

At the more expensive end of the market - £9.5m - is Castle Cary, a holiday park near Creetown in Dumfries and Galloay with a "ruined castle at its heart".

From BBC • Mar. 28, 2026

Tuesday, OpenAI announced it was abandoning its video-production aspirations, giving Disney a graceful exit until the next model storms the castle.

From Salon • Mar. 26, 2026

She didn’t want to build a castle anymore.

From "Caterpillar Summer" by Gillian McDunn