Generals who had guarded the Great Walls.[Photo provided to China Daily]

A jigsaw puzzle

The book only contains 20,000 Chinese characters, as there is much more information hidden in the graphics.

A 1.5-meter-long map of the Great Wall built during the Ming Dynasty (1368-1644), drawn by Li and his colleagues, is another highlight of the book.

"There was no whole map of the Great Wall that was built in the Ming Dynasty before, so we thought it was worth making that effort," says Li.

"We had to go through all possible materials to find the exact location of each part of the Great Wall and mark it on the map.

"It was like a jigsaw puzzle, because each source or book may only have one or two clues to the location and we needed to find the other ones to solve the puzzle."

Three experts on the Great Wall were invited to review all the content of the book. "And as a participant of the meetings, I also learned new information about the Great Wall," Li says.