n[C]
[Date: 1500-1600; Language: French;Origin: citadelle, from Old Italian cittadella, from cittade 'city', from Latin civitas; CITY]
a strong fort (=small castle) built in the past as a place where people could go for safety if their city was attacked
= fortress
the citadel of sth
literary a place or situation in which an idea, principle, system etc that you think is important is kept safe
= stronghold
　the last citadel of freedom