n.
  - a large building, typically of the medieval period, which is (or was formerly) fortified as a stronghold.
 - Chess,  informal old-fashioned term for rook2.
 
v.
 Chess  move one's king from its original square two squares along the back rank towards a rook on its corner square which is simultaneously moved to the square passed over by the king.
 
  Phrase
  
    
      - castles in the air 
(or in Spain)
 visionary unattainable schemes; daydreams. 
    
   
  Derivative
  
  Etymology
  OE: from Anglo-Norman Fr. and Old North. Fr. castel, from L. castellum, dimin. of castrum ‘fort’.