The winter chill that grips Beijing in December doesn't slow things down at all for Cheng Yongmao. He reaches his office in Huairou district in the capital's northeastern suburbs at 6:30 am and gets right down to business. "It's a force of habit," says Cheng, who's in his 60s and has been repairing and maintaining the Great Wall's Jiankou section in Huairou for nearly two decades.

Jiankou is widely considered one of the most dangerous parts of the Great Wall in Beijing and is known by hikers as the "wild Wall". The only way up to the bulwark is a rocky path along cliffs.

The Great Wall has been subject to natural disasters and human impact after standing for more than 450 years, and now direly needs maintenance and reinforcement.

"Construction work is generally conducted from April to October, and now it's time to prepare documents for future work," Cheng says.

Previous work has to be reviewed, and historical items unearthed during the cleaning and repairs need to be properly put in files, while construction charts for future endeavors have to be readied, he explains.

Cheng's office is filled with traces of his dealings with the Great Wall. A dozen walking sticks he made out of branches stand in the corner behind the door, next to a pile of tools he uses for tiling.