Emily Dickinson Lexicon

CAS'TLE, n. [kas'l; Sax. castel; L. castellum, from castrum; D. kasteel; Arm. gastell; Norm. chaxtel; Fr. château; Port. castello; It. id.; W. cast, envelopment, from câs, a being separated or insulated, hatred, envy, a castle; castell, a castle, whence castellu, to surround; casul, a cloke, a chasuble. The Welch câs gives the primary sense, which is to separate, to drive off; hence, to defend. It is probably from this root the Latins had casa. We observe in the Welch, câs signifies, separated, a castle, and hatred, envy; also, hateful, odious; and casnawr, a hater, a persecutor; casnori, to persecute, to chase. Hence we see the radical sense of hatred is a driving off.]

1. A house fortified for defense against an enemy; a fortress. The term seems to include the house and the walls or other works around it. In old writers, the word is used for a town or village fortified.
2. The house or mansion of a nobleman or prince.
3. In a ship, there are two parts called by this name; the forecastle, a short deck in the fore part of a ship, above the upper deck; and the hindcastle, at the stern. Castle in the air, a visionary project; a scheme that has no solid foundation.