don·jon /ˈdɑnʤən, ˈdʌn-/
  城堡的主樓
  Don·jon n.  The chief tower, also called the keep; a massive tower in ancient castles, forming the strongest part of the fortifications. See Illust. of Castle.
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  donjon
       n : the main tower within the walls of a medieval castle or
           fortress [syn: keep, dungeon]