The branches were probably used to reinforce the foundation, as they were found below the rammed-earth slope protection of the city wall and above a layer of soil, says Yue Hongbin, head of the institute.

"It indicates there was a pit of soft soil underneath when the wall was built. Without tree branches, the rammed earth of the wall would have sunk or cracked," Yue says, hailing the ancient wisdom behind the design.

Researchers say the branches are relatively well-preserved as they are buried 10 meters underground and protected by the red clay soil created by flooding of the Yellow River.