(CNN) -- When the last emperor of China fled Beijing's Forbidden City in 1924, the doors closed on one of its greatest treasures: the Qianlong Garden.
A secluded compound of pavilions and gardens built in the 1770s for the retirement of the Qianlong Emperor, it housed some of the most extravagant interiors found anywhere in the imperial palace complex.
As other areas were opened up to tourists, the garden remained mothballed for almost 100 years, its exquisite design and decorative treasures staying relatively unaltered since the 18th century.
Now some of those treasures are finally seeing the light of day at an exhibition at the Milwaukee Art Museum, Wisconsin from June 11, while the gardens undergo a $25 million restoration.
"Emperor Qianlong's reign is considered the zenith of the Qing dynasty and some of the highest prices fetched at auction today are for objects from that period," said historian Nancy Berliner, who curated the exhibition -- titled The Emperor's Private Paradise.
"It was an incredibly prosperous time and this was an emperor who was fascinated in the arts," she continued. "He pushed people to do their finest work, and that can be seen in the garden."
Items on view as part of the show -- a touring exhibition that opened at the Peabody Essex Museum in Salem -- include a magnificent throne made from the finest imported tropical hardwood inlaid with jade and semi-precious stones, and a spectacular Buddhist panel, painted on silk and glittering with gold, that depicts the universe and its deities in 2D and 3D.
A huge monumental jade and lacquer screen shows the 16 disciples of the Buddha, and was prized from a wall for the exhibition -- unintentionally revealing luxuriously decorated botanical images in gold lacquer on the back.
European art was among the Emperor's interests, and he enlisted the help of Jesuit missionaries to train his workshops in the use of Western perspective in painting. One of the highlights of the exhibition is a 12-foot trompe l'oeil mural depicting women and children in a palace hall celebrating the New Year. The mural is one of only eight such surviving works from the 18th century -- five of which are in the garden, according to Berliner.
"Qianlong is so important because it's the only still existent 18th-century interior garden that remains preserved as it originally was and not rebuilt or redesigned at any time," she said. "Most of the 18th-century gardens have either been destroyed or changed a lot over time, but because this was a small and private place it escaped the renovations that happened elsewhere in the palace complex."
Conservation of the Qianlong Garden began in 2002 as a joint project between the Palace Museum and the World Monuments Fund (WMF), and will be completed in 2019. It is the largest conservation project in the WMF's history, with just one of the garden's 27 buildings -- the aptly named Studio of Exhaustion from Diligent Service -- restored so far.
The Studio includes a private theatre, reception room and rooms for practising arts such as calligraphy and poetry. Three other buildings are slated to be finished in 2013.
One of the reasons the restoration is so time-consuming is that the Emperor spared no expense on his pet project -- applying intricate techniques usually reserved for fine art objects to whole walls and ceilings.
"You might find a room where there are decorations in very fine, hand-painted porcelain embedded into the screens, or others where fine lacquer -- usually used for furniture -- covers the whole room," said Henry Tzu Ng, WMF executive vice president. "The emperor took these fine art techniques and exploded them to an architectural scale."
The other main challenge is that the traditional materials and craftsmanship used to create the interiors are no longer readily available in modern China.
"Some techniques were not in use anymore, such as inner-bamboo-skin carving -- where bamboo stalks are soaked, flattened, turned into sheets of paper and poured over a mould to create landscapes like mountains or rockeries," said Ng.
When the project began, the Palace put out an appeal in the press calling for people who had knowledge of traditional craft techniques to come forward. As a result, they managed to track down artisans scattered in towns and villages around China whom they brought to Beijing to help with the restoration.
Once the work is complete, the external areas of the garden will be opened to the public -- but its opulent interiors will only be visible through large viewing glasses or on restricted tours, according to Ng.
The exhibition is therefore the only time people will be able to walk freely amidst the emperor's murals and artworks. "This opportunity won't happen again," said Ng. "Our goal is to put everything back where it was originally found."