Throughout Chinese history, enormous royal palaces were built for the emperors, their families, concubines, and entourage that survive today only as the rammed-earth base that supported elaborate timber halls (the oldest surviving wooden building in China is an 8th-century Buddhist temple hall of the Tang Dynasty). Rammed earth sections of the Great Wall, complete with ruined towers dotting the northern landscape to this day, date back to the Han Dynasty (202 BC - 220 AD). Likewise, the oldest surviving stone structures date back to the Han Dynasty (i.e. the monumental pillar gates outside tombs and shrines that mimic timber architectural styles, even having stone-carved representations of ceramic tiles). Archways, vaulted roofs, and domes built with bricks were restricted to tomb architecture during the Han period. Although monumental wooden Buddhist pagodas preceded them (as is known by written accounts), permanent stone and brick pagoda towers were first built during the Southern and Northern Dynasties, beginning with the Songyue Pagoda built in 523 AD by the Northern Wei in what is now Henan province.
Buddhist pagoda towers and imperial palaces - like Beijing's Forbidden City dating to the Ming Dynasty (1368-1644) - are usually thought of as the grandest and most monumental forms of Chinese architecture aside from stone arch bridges like Anji Bridge or the Great Wall of China. There is also monumental statuary; the largest stone-carved Buddha in the world is still the Leshan Giant Buddha found in modern Sichuan province, constructed from 713 to 803 AD during the Tang Dynasty and soaring to a height of 71 meters (233 ft). However, in my humble opinion, the greatest architectural feat achieved by Imperial China was a unique fusion of Chinese and Tibetan architectural styles during China's last imperial dynasty, the Qing (1644-1912).
I give you the Putuo Zongcheng Temple of what is now modern Chengde, Hebei province, constructed from 1760 to 1771 during the Qianlong Emperor's reign. It was modeled after the Potala Palace of Tibet (historical home of the Dalai Lama), although it contains many distinct Chinese architectural features such as an ascending succession of gatehouses that mirror the layout of Chinese imperial palaces. The final main structure looks very much like the Potala Palace, although it is crowned with Chinese-style Buddhist halls and pavilions.
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The Qianlong Emperor had a number of Buddhist temples throughout China built in this particular Sino-Tibetan style, although I've never come across one that is so massive as this. It has to be the most impressive Chinese temple complex I've ever seen, and simply rivals the grandeur of even the Forbidden City. Perhaps it speaks to the Manchu rulers' close relationship with the Tibetan lamas and their brand of Buddhism, which was already accepted wholesale by the nomadic Mongol tribes that surrounded Qing China.