Citadel

[3] The citadel of the Greek city of Mycenae was built atop a highly-defensible rectangular hill and was later surrounded by walls in order to increase its defensive capabilities.

[4] In Ancient Greece, the Acropolis, which literally means "high city", placed on a commanding eminence, was important in the life of the people, serving as a lookout, a refuge, and a stronghold in peril, as well as containing military and food supplies, the shrine of the god and a royal palace.

When finally gaining possession of the place, the Maccabeans pointedly destroyed and razed the Acra, though they constructed another citadel for their own use in a different part of Jerusalem.

At various periods, and particularly during the Middle Ages and the Renaissance, the citadel – having its own fortifications, independent of the city walls – was the last defence of a besieged army, often held after the town had been conquered.

This was used, for example, during the Dutch Wars of 1664–1667, King Charles II of England constructed a Royal Citadel at Plymouth, an important channel port which needed to be defended from a possible naval attack.

[10] In the 19th century, when the political climate had liberalized enough to permit it, the people of Barcelona had the citadel torn down, and replaced it with the city's main central park, the Parc de la Ciutadella.

The attack on the Bastille in the French Revolution – though afterwards remembered mainly for the release of the handful of prisoners incarcerated there – was to considerable degree motivated by the structure's being a Royal citadel in the midst of revolutionary Paris.

The Siege of the Alcázar in the Spanish Civil War, in which the Nationalists held out against a much larger Republican force for two months until relieved, shows that in some cases a citadel can be effective even in modern warfare; a similar case is the Battle of Huế during the Vietnam War, where a North Vietnamese Army division held the citadel of Huế for 26 days against roughly their own numbers of much better-equipped US and South Vietnamese troops.

[13] Since the mid 20th century, citadels have commonly enclosed military command and control centres, rather than cities or strategic points of defence on the boundaries of a country.

The military citadels under London in the UK, including the massive underground complex Pindar beneath the Ministry of Defence, are examples, as is the Cheyenne Mountain nuclear bunker in the US.