A citadel is a fortress for protecting a town, sometimes with a castle in its middle. Citadels are most often used to protect a garrison or political power from the inhabitants of the town it is defending. They were designed to ensure loyalty from the town which they defended.

In a fort with bastions, the citadel is the strongest part of the fort, well inside of the outer walls and bastions.

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Citadel is a record label based in Sydney, Australia that was responsible for releasing records by The Stems, The Lime Spiders, and Died Pretty.

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The Citadel is a public military college in Charleston, South Carolina.

The Citadel is a military installation and vice-regal residence in Quebec City, Quebec; see Le Citadelle.

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Citadel is also the name of a BBS software which was developed in Seattle, Washington.