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.

• Citadel Records is an Australian independent record label based in Sydney that released records by The Stems, The Lime Spiders and Died Pretty.
• 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.
• Citadel is the name of a BBS software which was developed in Seattle, Washington.
• Citadel is also the name of a miniatures company owned by Games Workshop
• Citadel and its sequel Citadel 2 were arcade adventure games created for the BBC Micro and Acorn Electron computers by Superior Software in the 1980s.