From Longman Dictionary of Contemporary Englishcitadelcit‧a‧del /ˈsɪtədəl, -del/ noun [countable] 1 PMa strong fort (=small castle) built in the past as a place where people could go for safety if their city was attacked SYN fortress2 → the citadel of something
Examples from the Corpus
citadel• Outside this was a citadel, fortified like the inner bailey, but containing a greater number of buildings.• Whenever there was fighting around Limoges one permanent complicating factor was the rivalry between city and citadel.• With your help, and that of the Forteviot men, the main citadels may still be standing.• Far from being a challenger for power, it could not even hold on to its old citadels.• It seemed that the verderers were going towards the outlaws' citadel, not away from it.• New Zealand became the citadel of the new right.• In the citadel of St Martial everyone's nerves were on edge.Origin citadel (1500-1600) French citadelle, from Old Italian cittadella, from cittade “city”, from Latin civitas; → CITY 