Source : Webster's Revised Unabridged Dictionary (1913)

Citadel \Cit"a*del\, n. [F. citadelle, It. citadella, di?. of
   citt? city, fr. L. civitas. See {City}.]
   A fortress in or near a fortified city, commanding the city
   and fortifications, and intended as a final point of defense.

   {Syn}. -- Stronghold. See {Fortress}.

Source : WordNet®

citadel
     n : a stronghold into which people could go for shelter during a
         battle [syn: {bastion}]