Babak Fortress , also known as the Immortal Castle or Republic Castle, is a large citadel on the top of a mountain in the Arasbaran forests, which is located 16 km southwest of Kalibar City in northwestern Iran, Azarbaijan province. The fortress was built during the Sasanid dynastic era; however it is famous for the 23 years residence ( 816 to 837 CE) against the Arab invaders by Persian hero Babak -e- Khoramdin and his warriors. Eventually Babak, his wife, and his warriors were forced to leave their command post at the castle under very difficult circumstances. He was betrayed and was handed over to the Arab invaders. 
Every year at the last week of June (At first week of summer by Iranian Calendar) Babak's birthday symbolically is celebrated at the castle.