Pāpak Fort or 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.
The castle was constructed in two floors and as you enter the castle you can see the main hall which is surrounded by seven rooms. On the east part of the castle, you can see cisterns and during the winter, it used to be filled with snow and rain and were used under siege.
The castle, built on 2300–2600 meter heights, surrounded by 400 to 600 meter gorges, is accessed via a long series of broken steps that lead to the top of a hill. Thereafter, the easiest route is a long dirt track. No signs lead to the Castle. At the end of the dirt track, the route turns left. The first sign of the ruins appears on the left, leaving two peaks to cross. The first peak has views of the Castle. After ascending a second peak, with additional ruins, the trail passes sheer cliffs on the right with no railings. From the castle, the surrounding Arasbaran oak forest, jagged cliffs, mountains in the distance, and Iranian history combine inform the visitor's perspective. The last stretch is a narrow passageway and a 200 meter corridor-shaped temple.