A citadel is a fortified structure or stronghold designed to protect a city or settlement from attacks. Typically built on elevated ground, citadels serve as a refuge for inhabitants during times of conflict. They often feature thick walls, towers, and strategic vantage points to defend against invaders. Historically, citadels were essential in military architecture, providing safety and a place for troops to gather. Many ancient cities, such as Jerusalem and Athens, had citadels that played crucial roles in their defense. Today, some citadels have been preserved as historical sites, attracting visitors interested in history and architecture.