Source : Webster's Revised Unabridged Dictionary (1913)

Donjon \Don"jon\, n. [See {Dungeon}.]
   The chief tower, also called the keep; a massive tower in
   ancient castles, forming the strongest part of the
   fortifications. See Illust. of {Castle}.

Source : WordNet®

donjon
     n : the main tower within the walls of a medieval castle or
         fortress [syn: {keep}, {dungeon}]