The Bohai State was a local regime set up by the Sumo clan of Mohe people in Chinese northeast area in the Tang Dynasty (618-907). It established five capitals successively, with the Shangjing Longquanfu as its main capital.
Surrounded by mountains on four sides, the city was embraced by the Mudan River on three sides. During the last year of Emperor Xuanzong of the Tang Dynasty, King Wen of the Bohai State moved the capital here, and then to the Dongjing Longyuanfu in the first year of the Zhenyuan reign. In the 10th year (794) of the Zhenyuan reign, King Cheng moved the capital back here, and the city remained the capital till the Bohai State was conquered by Emperor Taizu of the Liao Dynasty (916-1125) in the first year (926) of the Tianxian reign. The Archaeology Research Institute of the Chinese Academy of Social Science conducted a large-scale excavation during 1963-1964, and revealed the planning and construction style of the city.
The construction style of Shangjing City imitated that of Chang'an City, which was divided into three sections, namely the inner city, the outer city and the palace area. Taking a shape of rectangle, the city had a perimeter of 17.5 kilometers. Surrounded by moats, the city walls, about 2.4 meters thick, were built with stones. 10 gates were opened on the four sides of the city, with the north and south sides having 3 gates each while the east and west 2 gates each, which were built symmetrically. Inside the city were 5 streets stretching from east to west, and 3 streets from the south to north, all went a straight line. The grid-pattern streets divided the whole city into a number of regular rectangular areas that were built into lanes with stone walls as the separation. Most of the lanes had two sides directly open into the streets.
With a perimeter of about 2.5 kilometers, the inner city was built in the middle of the north part of the outer city. The remains of the city walls were about 3 meters high.
The palace was located in the center of the inner city and took a rectangular shape. Surrounded by stone walls, it was 1,390 meters long from the south to the north, and 1,050 meters wide from the east to the west, with a gate open on each side. Inside the palace, foundations of 5 halls were constructed on the axis line stretching from south to north. The first two halls were used for ceremonies and meetings. Larger than the other ones, these two halls had thick stone foundations decorated with stone carving hornless dragon head. The floor was covered with quadrels with decorative patterns, and the column base was made of green glazed pottery. The roof was covered with gray tiles together with green glazed tiles. The roof ridge was decorated with green-glazed hawk tails and beast heads. Comparatively small in size, the other 3 halls were believed to be bedrooms, which were equipped with warming facilities, such as channels to transmit heat to heatable brick bed and chimneys.
Lying to the east of the palace was an imperial garden, with a 20,000-square-meter pond built to its south and 2 pavilions facing each other to its north. The pavilions only had pillar remains left. On the east and west sides of the pond were artificial hills and some pavilion remains.
The Site of the capital of the Bohai State has offered important material objects for the study of the history of the Bohai State as well as the history of city and architectural developments of the Tang Empire.