This article is about a type of fortification. For other meanings of this word, please see: Citadel (disambiguation).

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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.