The Citadel of Aleppo is a large medieval fortified palace in the centre of the old city of Aleppo, northern 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. An extensive conservation work has taken place in the 2000s by the Aga Khan Trust for Culture in collaboration with Aleppo Archeological Society.