Bābak Fort or Babak Castle (Persian: دژ بابک or قلعه بابک Azerbaijani: بابک قلعه سي ), 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.