A citadel is a large fortress mainly used to protect towns or castles from attacks or disaster, though citadels have been built for other reasons as well. The oldest known citadels were built in the Indus Valley Civilization, where the citadel may have been used as a symbol of authority, no one is fully sure about their purpose.
In Ancient Greece, the Acropolis was important in the life of the people, serving as a refuge and stronghold in peril and containing military and food supplies, the shrine of the god and a royal palace. The most well-known is the Acropolis of Athens, but nearly every Greek city-state had one - the Acrocorinth famed as a particularly strong fortress.
The Citadelle of Quebec still survives as the largest citadel still in official military operation in North America after more than two hundred years of existence.