cit ‧ a ‧ del /ˈsɪtəd ə l, -del/ BrE AmE noun [countable]
[ Date: 1500-1600 ; Language: French ; Origin: citadelle , from Old Italian cittadella , from cittade 'city' , from Latin civitas ; ⇨ ↑ city ]
1 . a strong ↑ fort (=small castle) built in the past as a place where people could go for safety if their city was attacked SYN fortress
2 . the citadel of something literary a place or situation in which an idea, principle, system etc that you think is important is kept safe SYN stronghold :
the last citadel of freedom