The Citadel of Aleppo is a large medieval citadel located in the center of the old city of Aleppo, Syria. It is considered to be one of the oldest and largest castles in the world. Usage of the Citadel hill dates back at least to the middle of the 3rd millennium BC. Subsequently occupied by many civilizations including the Greeks, Byzantines, Ayyubids and Mamluks, the majority of the construction as it stands today is thought to originate from the Ayyubid period. The citadel was inscribed as a UNESCO World Heritage Site in 1986 as part of the ancient city of Aleppo.