This enabled him to take part in many key restoration projects, such as repairing the roofs of the Beijing West Railway Station and of the former residence of luminary Chinese writer Lu Xun.

Cheng received his first Great Wall assignment in 2004. It was for the Huanghuacheng section in Huairou.

"Working on the mountain ridges poses extra challenges, since everything is irregular," he says.

"You need to look around whenever you lay a tile, taking into account such factors as gradient changes … The mules can carry the repair materials to the foot of the Great Wall, but we still need to carry them up to the (construction) sites ourselves."

He was head of a construction team of dozens of members, most of whom came from mountainous areas in neighboring Hebei province and were consequently good at trekking up and down the peaks.

Still, some places were too dangerous. Ordinary people couldn't even safely stand, let alone work, in these areas, he recalls.

Some workers' feet hurt so much that they couldn't walk properly. But under Cheng's leadership, they adapted to the challenging environment and built up their strength.

As his Great Wall experience increased and relevant aptitudes improved, Cheng came up with a five-principle restoration approach.

First, no disorderly or overlapping layers should emerge amid the original stone, brick and soil. Then, restoration can take place in accordance with the slopes' conditions, with attention given to angles.

The other principles are about following the original mountain bends, and repairing the Wall only as necessary, and remaining faithfully to its historical context, Cheng explains.