In the east of the city of Birjand and five kilometers from the city of Darmyan , in the village of Forg, there is a castle called Furge, which is of great importance to the Ismailis after the castle of Alamut. The castle was built at the time of Nadir Shah and by Mirza Baqa Khan, the ruler of the area, and his son Mirza Rafi Khan finished it. The castle is erected on top of rocky rock with materials made of stone, brick, clay, plaster, lime and saroux.
The interior of the castle is composed of various parts such as the residence of the crew, the stables, ammunition depots, and so on, surrounded by circular towers of the castle. The top of the fence and towers has jaged and there is a pore to watch and defend. The main gate is on the east side and the smaller gate is built on the northern side. The king is at the highest point, where the stables, the water storage, the two-story rooms and the mosque are built in this part. With regard to the works and architectural remnants of the castle, it is concluded that the castle has been restored in several stages and has been enlarged.