A strong fortress, usually built on high ground, that protects a city. It is the last place of defense if the rest of the city is attacked.
From French "citadelle," from Italian "cittadella," a diminutive of "città" meaning "city," from Latin "civitas." It originally meant a "little city"—a defended core inside or beside the main city.
A citadel is like a city’s emergency backup city, built to survive when everything else fails. Even today we use "citadel" metaphorically for the safest, most protected part of something—like a mental or cultural fortress.
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