The Collaborative International Dictionary of English v.0.48:

Donjon \Don"jon\ (d[u^]n"j[u^]n), 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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

donjon
    n 1: the main tower within the walls of a medieval castle or
         fortress [syn: keep, donjon, dungeon]