The Mithraeum of the Zerzevan Castle is the most important structure remaining. It was built on the north end of the walls by carving the main rock into the underground. At the entrance gate of the structure, the inscriptions and symbols are clearly visible. On the eastern wall of the structure, there are columns carved into the main rock, two large niches in the middle and two small niches on the both sides. A bull sacrifice scene is carved on the plaque in the middle big niche. Paint residues can be seen on the belt rising above the two columns around the big niche in the middle.