Source : Webster's Revised Unabridged Dictionary (1913)

   2. Any strong, imposing, and stately mansion.

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

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

   {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}.

Source : WordNet®

castle in Spain
     n : absent-minded dreaming while awake [syn: {reverie}, {revery},
          {daydream}, {daydreaming}, {oneirism}, {air castle}, {castle
         in the air}]