Other forms: castles, castling
A castle is a huge stone building with thick walls where lords once lived. A ruined castle, castle walls, the castle keep.
It is a fortified residence, which is what separates it from a fort — somebody lived there. Britain has well over a thousand in some state. In chess, to castle is a move putting the king behind a rook. Note an Englishman's home is his castle; and that castle and chateau are the same Latin word arriving twice.
1nouncastleA fortified building where lords once lived.
“The king and queen lived in a castle on top of the hill.”
2verbchessTo make a chess move putting the king behind a rook.
“In chess, it's often wise to castle early and keep your king safe.”
Too few real examples of this word to estimate which sense is commonest.
castle is a fortified building.
Latin castellum is a diminutive of castrum, a fort. The French chateau is the same word, and the -chester in English place names comes from castrum too.
Middle English
castel
English, today
castle
Built from
From Middle English castel. From Middle English castle, castel, from late Old English castel, castell (“a town, village”), borrowed from Latin castellum (“small camp, fort”), diminutive of Latin castrum (“camp, fort, citadel, stronghold”). a doublet of cashel and castell and castellum and château. Parallel borrowings (from Late Latin or Old French) are Scots castel, castell (“castle”), West Frisian kastiel (“castle”), Dutch kasteel (“castle”), German Kastell (“castle”), Danish kastel (“citadel”), Swedish kastell (“citadel”), Icelandic kastali (“castle”), Welsh castell.
Etymology from Wiktionary, CC BY-SA