Zhou Bing still feels moved when he recalls entering the long-sealed-off retirement lodge of Emperor Qianlong (1711-99) - Juanqinzhai, the most mysterious and best preserved of Forbidden City structures, which remains off-limits to the public.

Zhou says the only thing more inspiring is being able to share this with the world in his newest documentary, The Emperor's Secret Garden.

Zhou Bing, director of documentary The Emperor's Secret Garden. Provided to China Daily

Juanqinzhai was Qianlong's retirement gift to himself.

Its construction conscripted the best craftspeople of the time, making it the period's crown jewel of interior design.

Zhou's documentary records the repair of Juanqinzhai by the Palace Museum and the World Monuments Fund (WMF).

It aired on Sky TV in the United Kingdom and was shown at the Palace Museum in April. China Central Television (CCTV) and Beijing Television will show it in the coming two months. The DVD will be released after the broadcasts.

Shooting began in 2007, a few years after the restoration began.

But Zhou had been filming the imperial palace since 2003, when he began preparing his 12-episode documentary, The Forbidden City, which was a nationwide hit in 2005.

He believes his latest documentary was fateful.

"The first scene I shot for The Forbidden City was right outside Juanqinzhai," the 44-year-old recalls.

Zhou was familiar with the building but was excited to experience some of the behind-the-scenes action, which provided him with knowledge he "had no way of knowing if not for shooting the documentary".

He was especially intrigued by the Palace Museum's efforts to find the handful of unknown craftspeople, who sustained dying forms of craftsmanship used in Qianlong's lodge, or were able to revive previous practices.

"People from older eras spent many years on one task without commercial purpose," Zhou says.

"We've lost that spiritual quest in this age. I'm glad to see it again in the restoration."

And the documentary is based on the same principle, he says.

"It's a matter of conscience," he says.

"I consider it my mission to visually document China's achievements for future generations."