citadel
[noun]
Synonyms: fortress, bastion, fortification, keep, stronghold, tower

[TahlilGaran] English Synonym Dictionary ▲

cit‧a‧del /ˈsɪtədəl, -del/ 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 Synonym : 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 Synonym : stronghold:
the last citadel of freedom

[TahlilGaran] Dictionary of Contemporary English ▲