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

Citadel's view from the top of an Irish castle

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 fortification with bastions, the citadel is the strongest part of the system, sometimes well inside of the outer walls and bastions, but often forming part of the outer wall for the sake of economy. It is positioned to be the last line of defense should the enemy breech the other components of the fortification system.