The Citadel reference article from the English Wikipedia on 24-Apr-2004
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Citadel

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Citadel's view from the top of an Irish castleEnlarge

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