The Collaborative International Dictionary of English v.0.48:

Castle \Cas"tle\, n. [AS. castel, fr. L. castellum, dim. of
   castrum a fortified place, castle.]
   1. A fortified residence, especially that of a prince or
      nobleman; a fortress.
      [1913 Webster]

            The house of every one is to him castle and
            fortress, as well for his defense againts injury and
            violence, as for his repose.          --Coke.
      [1913 Webster]

            Our castle's strength
            Will laugh a siege to scorn.          --Shak.
      [1913 Webster]

   Note: Originally the medi[ae]val castle was a single strong
         tower or keep, with a palisaded inclosure around it and
         inferior buidings, such as stables and the like, and
         surrounded by a moat; then such a keep or donjon, with
         courtyards or baileys and accessory buildings of
         greater elaboration a great hall and a chapel, all
         surrounded by defensive walls and a moat, with a
         drawbridge, etc. Afterwards the name was retained by
         large dwellings that had formerly been fortresses, or
         by those which replaced ancient fortresses. A Donjon or
         Keep, an irregular building containing the dwelling of
         the lord and his family; B C Large round towers ferming
         part of the donjon and of the exterior; D Square tower,
         separating the two inner courts and forming part of the
         donjon; E Chapel, whose apse forms a half-round tower,
         F, on the exterior walls; G H Round towers on the
         exterior walls; K Postern gate, reached from outside by
         a removable fight of steps or inclined plane for
         hoisting in stores, and leading to a court, L (see
         small digagram) whose pavement is on a level with the
         sill of the postern, but below the level of the larger
         court, with which it communicates by a separately
         fortified gateway; M Turret, containing spiral stairway
         to all the stories of the great tower, B, and serving
         also as a station for signal fire, banner, etc.; N
         Turret with stairway for tower, C; O Echauguettes; P P
         P Battlemants consisting of merlons and crenels
         alternately, the merlons being pierced by loopholes; Q
         Q Machicolations (those at Q defend the postern K); R
         Outwork defending the approach, which is a road
         ascending the hill and passing under all four faces of
         the castle; S S Wall of the outer bailey. The road of
         approach enters the bailey at T and passes thence into
         the castle by the main entrance gateway (which is in
         the wall between, and defended by the towers, C H) and
         over two drawbridges and through fortified passages to
         the inner court.
         [1913 Webster]

   2. Any strong, imposing, and stately mansion.
      [1913 Webster]

   3. A small tower, as on a ship, or an elephant's back.
      [1913 Webster]

   4. A piece, made to represent a castle, used in the game of
      chess; a rook.
      [1913 Webster]

   Castle in the air, a visionary project; a baseless scheme;
      an air castle; -- sometimes called a castle in Spain (F.
      Ch[^a]teau en Espagne).

   Syn: Fortress; fortification; citadel; stronghold. See
        Fortress.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Castle \Cas"tle\, v. i. [imp. & p. p. Castled; p. pr. & vb. n.
   Castling.] (Chess)
   To move the castle to the square next to king, and then the
   king around the castle to the square next beyond it, for the
   purpose of covering the king.
   [1913 Webster]

WordNet (r) 3.0 (2006):

castle
    n 1: a large and stately mansion [syn: palace, castle]
    2: a large building formerly occupied by a ruler and fortified
       against attack
    3: (chess) the piece that can move any number of unoccupied
       squares in a direction parallel to the sides of the
       chessboard [syn: castle, rook]
    4: interchanging the positions of the king and a rook [syn:
       castle, castling]
    v 1: move the king two squares toward a rook and in the same
         move the rook to the square next past the king

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "castle":
   acropolis, bastion, beachhead, bishop, blockhouse, bridgehead,
   bunker, chateau, chessman, citadel, court, donjon, fasthold,
   fastness, fort, fortress, garrison, garrison house, hall, hold,
   keep, king, knight, man, manor, mansion, martello, martello tower,
   mote, motte, palace, palais, palatial residence, palazzo, pawn,
   peel, peel tower, piece, pillbox, post, queen, rath, rook,
   safehold, strong point, stronghold, tower, tower of strength,
   villa, ward



Easton's 1897 Bible Dictionary:

Castle
   a military fortress (1 Chr. 11:7), also probably a kind of tower
   used by the priests for making known anything discovered at a
   distance (1 Chr. 6:54). Castles are also mentioned (Gen. 25:16)
   as a kind of watch-tower, from which shepherds kept watch over
   their flocks by night. The "castle" into which the chief captain
   commanded Paul to be brought was the quarters of the Roman
   soldiers in the fortress of Antonia (so called by Herod after
   his patron Mark Antony), which was close to the north-west
   corner of the temple (Acts 21:34), which it commanded.


U.S. Gazetteer Places (2000):

Castle, OK -- U.S. town in Oklahoma
   Population (2000):    122
   Housing Units (2000): 58
   Land area (2000):     0.176491 sq. miles (0.457110 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    0.176491 sq. miles (0.457110 sq. km)
   FIPS code:            12750
   Located within:       Oklahoma (OK), FIPS 40
   Location:             35.475174 N, 96.384072 W
   ZIP Codes (1990):     74833
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Castle, OK
    Castle