But its treasures are secret no more.
Artefacts from Emperor Qianlong's "hideaway" are on display at the Hong Kong Museum of Art in an exhibition titled A Lofty Retreat from the Red Dust.
The emperor had the garden built between 1772 and 1776 and named it after himself. The Qianlong Garden is located in the northeastern corner of the Forbidden City in Beijing.
Qianlong was one of the most famous emperors of the Qing dynasty. He ruled China from 1735 to 1796. The garden was reserved as a private retreat for the emperor. Its design incorporated traditional methods of craftsmanship and materials no longer used today.
The enclosure is home to four courtyards, rockeries, 27 pavilions and other structures.
The garden was closed after China's last emperor left the palace in 1924.
"Qianlong Garden is like a small treasure box; it contains the emperor's private collections, which allow people to take an intimate look into his life," says Rose Lee Wing-chong, the museum's curator of Chinese antiquities.
"The exhibit will allow you to see another side of Qianlong as you revisit the history."
Emperor Qianlong was known to take interest in foreign cultures and even allowed Jesuit missionaries into his court. One foreigner was Italian artist Giuseppe Castiglione, whose paintings are among the exhibits. In one, Castiglione painted a portrait of the emperor riding a horse, with traditional Chinese style background scenery drawn by local artists.
Lee says the exhibits have been arranged in line with themes reflecting the emperor's desires.
The first section reflects Qianlong's wish for retirement and a scholarly life through his own calligraphy and poems. It features a portrait of him dressed as a scholar.
The second includes exhibits with themes of children or deer, representing longevity.
The third pays homage to the emperor's strong faith in Buddhism.
The final section showcases the emperor's passion for the arts.
The exhibition is packed with interactive features which offer visitors an unforgettable experience. A digital version allows you to take a virtual tour of the garden. Interactive games also let you build and decorate your own garden.
The Hong Kong exhibit is sponsored by The Hong Kong Jockey Club Charities Foundation. It boasts 75 sets of paintings and calligraphy, furniture, mural paintings, architectural elements and religious art. They are on loan from Beijing's Palace Museum.
The collection arrived here on June 21, in time for the 15th anniversary of the formation of the HKSAR government.
A Lofty Retreat from the Red Dust: The Secret Garden of Emperor Qianlong will be on show until October 14. For more information, visit their website.