This paper looked into the structure of and changes in the building process of Gijang mountain fortress and its receiving reservoir by analyzing data from an excavation investigation. The structure of the receiving reservoir may be classified into a flat form, stone sheath, floor facility, wall facility, and entry and exit facility. The flat form of the Gijang mountain fortress and receiving reservoir is round. Concerning the sectional form, the wall was obliquely excavated in the trapezoid. As a stone sheath building method, it was built by undertaking a range work of oblong stone materials in a clockwise direction on a stamped soil floor. Then, it was treated with stamping using double layers of gray clay and yellowish brown clay on the floor and the wall. Also, in a space between the stamped layers on the floor, herbal plants and a straw mat were laid for waterproofing as well as to prevent sinking. As an entry and exit facility, two facilities were confirmed symmetrically in the southeast and in the northwest. It is believed that they were built additionally during rebuilding after the initial construction. The building process was revealed to have been carried out in 8 stages. Given the structure and excavated remains, the building period is estimated to be the early to mid 7th century for the initial building, the later 9th to 11th centuries for the primary rebuilding, and the later 16th to early 17th centuries for the secondary rebuilding.