A citadel is a fortified structure or stronghold that is typically built to protect a city or important location from attacks. These structures often feature thick walls, towers, and strategic positions to provide a defensive advantage. Citadels have been used throughout history in various cultures, serving as military bases, royal residences, or centers of governance. In addition to their military function, citadels often symbolize power and authority. Many ancient cities, such as Jerusalem and Carcassonne, have well-preserved citadels that attract tourists and historians alike. Today, some citadels are recognized as UNESCO World Heritage Sites, highlighting their cultural and historical significance.