Wang points out: "Juanqinzhai's exterior is quite simple, but its interior is unrivaled."

The most dazzling decorations are the 170 square meters of paint-on-silk panoramic murals that cover the ceiling and walls of the lodge's theater.

More than 20 works of various sizes infuse Chinese materials and subjects with Western techniques of perspective and trompe l'oeil, which was introduced by Italian artist and missionary Giuseppe Castiglione (1688-1766), who served as Qianlong's court painter until the artist's death.

The panoramic murals are believed to have been created by Castiglione's student Wang Youxue.

The northern wall is adorned with red-crowned cranes, magpies and peonies - all auspicious symbols.

The painting of the Fuwangge (Pavilion of Expecting Good Omens) mirrors the actual pavilion seen through the southern wall's window.

"This is the Forbidden City's only surviving panoramic mural site and is China's largest," Wang says.

The theater contains a gold-leaf-covered indoor pavilion that serves as a stage with wooden railing.

Partition screens divide five rooms on the two-story area on the rectangular building's eastern side into uneven compartments, each with a cot so Qianlong could sit to meditate.

"Qianlong was creative in using such limited space in interesting ways," Wang says.

The partitions are made of red sandalwood carved and inlaid with bifacial embroidery, jade and bamboo thread marquetry. Many also feature inner bamboo-skin carvings created using a form of craftsmanship that's virtually dead today.

Wang, who led the project, says the structure was sound.

"Everything inside was in place but in a precarious state," he says.

Some materials had aged better than others.

The screens were mostly intact. But much of the inlaid jade and bamboo threads had fallen off, as the glue lost efficacy. The centuries had claimed most of the bifacial embroideries.

The repair team scoured several southern provinces to find inheritors of these nearly - or completely - dead forms of craftsmanship.

Another challenge was posed by the murals, which were damaged by worms, stains and fading.

The painting and calligraphy mounting section of the Palace Museum's Cultural Relics Protection and Technology Department teamed up with the WMF to revitalize the paintings.

"We took down all the pieces, repaired them and seamlessly pasted them back on the ceiling and walls without affecting the indoor stage or other displays," the section's associate researcher Yang Zehua says.

"It was one of the most challenging tasks we've ever done. I believe it's unique in the world."

In 2006, the project's success prompted expanded cooperation between the Palace Museum and the WMF to rejuvenate the Palace of Tranquil Longevity Garden, which Qianlong designed with four courtyards, 24 buildings, and plants and rockeries. The project is slated for completion in 2019.

"Every building in the Qianlong Garden has a different structure and interior design," Wang says.

"The historic interpretation and research of Juanqinzhai can't be separated from the rest of the garden."