Wat Phu in Champasak, Laos 

From there the sacred stream flowed down the artificially terraced mountain slope into two sacred reservoirs or barays and finally into the Mekong River, whose life-giving waters were believed to sustain the whole of the ancient Khmer Empire. Standing structures within the temple complex include quadrangles, a Nandin Hall, small pavilions, brick towers, stairways and the main shrine, which was dedicated to Shiva.
This Wat is the most important attraction of South Laos, although it is not Laotian in origin. Wat Phou was built by the Khmer.
Despite of being smaller, Wat Phou can be compared to Angkor. Not only has it been built in the same style; it also exudes the same atmosphere of an ancient city lost in the jungles for uncounted centuries. While the structures themselves are overpowering, it's the jungle overgrowth that creates a particular charm, at Angkor as well as Wat Phou. The Wat has stood unused for centuries because it has been built as a Hindu, not a Buddhist temple.
Wat Phou is older than Angkor. It was founded in the first decades of the 9th century by Jayavarman II, the first king of an independent Khmer nation. Before Jayavarman II, the Khmer were ruled by the Javanese (Indonesians). For some time Wat Phou served Jayavarman II as Khmer capital.
Though Jayavarman II soon moved his government to the Angkor plain, Wat Phou remained an important pilgrim's destination for the entire Angkor era (about 600 years).