Babak Castle, also known as the Immortal Castle or Republic Castle, is a large citadel and National Symbol of Iranians on the top of a mountain in the Arasbaran forests, which is located 6 km southwest of Kalibar City in northwestern Iran. It has been identified as the stronghold of Pāpak Khorramdin, the leader of the Khurramites in Azarbaijan who fought the Islamic caliphate of Abbassids. Iranian Azarbaijanis gather at Babak Castle during the first weekend in July for the annual commemoration of Babak Khorramdin.