donjon

from WordNet (r) 3.0 (2006)

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

from 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]