Kol Castle (Hassan Sabbah), also known as Mo'menabad Fortress, is located near the village of Taghandik in Darmian County. This castle is the most valuable and important Ismaili fortress in the historical region of Qohestan and the second defensive center after Alamut Castle. In the westernmost part of this castle, architectural remains in the form of a mound of earth can be seen atop the mountain. This area is connected to the central part by a narrow corridor about three meters wide. The central part is relatively better preserved than other parts of the castle, and its architectural remains are recognizable. The architectural elements of this section include corridors, rooms, and large halls made of stone. The ceilings of the rooms in this section are in the form of pointed arches. The third section is located in the east of the castle. In this part, the outer wall of the castle and several towers are recognizable. Scattered across the surface of the castle are numerous decorated bricks with animal and plant motifs. These bricks are square, rectangular, and hexagonal in shape and were used to decorate various parts of the castle. Scattered across the surface of the castle are pieces of Islamic glazed pottery in turquoise, blue, green, and yellow colors, as well as pottery with molded designs.