In a roof decoration from Chongyang Hall, one of the buildings in the Yongle Palace compound, researchers found a roll bearing the words "royal order". After being unpacked, it turned out to be a precious Quanzhen Taoist sutra, known as The Scripture of the Marvelous Treasure of the Highest Mystery of the Numinous Void, Inconceivable Salvation of Living Beings, and Supreme Excellence, and is listed among China's highest-level national cultural artifacts.

"It further proves the high status of Yongle Palace at its peak," An says.

The scroll is on display with other Quanzhen scriptures in a gallery in the national library, and offers visitors a glimpse into the age when Yongle Palace was a place of high-ranking pilgrims.

"Genghis Khan's westward expedition also resulted in cultural exchange, and Yongle Palace may be one example," the curator says, referring to the exhibited chiwen, an ornamental tile that decorates both ends of the ridgepole on the roof.