A citadel is a fortress built on high ground to protect a city, often serving as the last refuge if enemies break through the outer walls. Picture a massive stone stronghold on a hill overlooking a medieval town, where defenders could hold out even after the rest of the city fell. The word comes from Italian, meaning “little city,” because these fortresses were like cities within cities, with their own wells, food stores, and living quarters.

Throughout history, citadels gave cities their best chance of survival during war. The ancient Greeks built citadels called acropolises (the most famous being the Acropolis of Athens). Medieval castles often served as citadels. During a siege, when enemy armies surrounded a city, residents would retreat behind the citadel's thick walls and high position, making it extremely difficult for attackers to conquer.

Today, people use citadel metaphorically to describe any stronghold or center of power. Someone might call a university a “citadel of learning” or describe a company as the “last citadel” of a dying industry. The word suggests something powerful, defensible, and built to endure.