The five-metre-long and four-metre-wide room was built nearly one metre below the ground at the ruins of Yueyang City, capital of the Qin state during the Warring States Period from 476-221 BC.
Stone pillar bases and square bricks were also found in the room.
Archaeologist Li Yufang said buildings with basements were rare during that period, denoting an upscale style and indicating the ruins were actually the residential palace of the state ruler.
A fireplace - a feature only seen in residential palaces of rulers during the period - was also found at the ruins, said Liu Rui, a researcher with the Institute of Archaeology of the Chinese Academy of Social Sciences and head of the excavation team.
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The remains of Yueyang City are in the district of Yanliang in Xi'an city, Shaanxi Province in northwest China.
It served as the capital of the Qin state for 35 years.
A famed political reform took place in the city about 2,300 years ago, when Qin statesman Shang Yang initiated a series of reforms that led to the foundation of China's legal system.
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