Prasat Tamnak Sai is a small yet historically and architecturally significant Khmer sanctuary located in Ban Samrong Kiat, Khun Han District, Sisaket Province. It is believed to have been constructed around the 11th–12th century during the height_ of the Khmer Empire, influenced by the Khleang and later Baphuon styles of Khmer art, which were commonly used in religious architecture throughout Southern Isan.The sanctuary’s most distinctive feature is its single brick tower built on a laterite base with a square layout. The structure faces east in accordance with Hindu beliefs, where the east symbolizes divinity and auspiciousness. The tower has one real entrance on the eastern side, while the remaining three sides contain elaborately designed false doors, created for architectural balance and symbolic meaning.

One of the most fascinating aspects of the sanctuary is its ancient brick construction technique. The bricks were carefully arranged so tightly that almost no mortar joints are visible, demonstrating the advanced craftsmanship of Khmer builders of the period. Many bricks remain in remarkably good condition despite centuries of exposure, although the upper section of the tower has collapsed over time, leaving only the walls and door frames as evidence of its original form.In the past, the sanctuary was heavily surrounded by massive banyan trees whose roots wrapped around the brick structure, giving the impression of a hidden royal pavilion within the forest. This unique appearance inspired the local name “Prasat Tamnak Sai,” meaning “The Banyan Pavilion Sanctuary.” Although some of the trees have since been trimmed for conservation purposes, the site still retains its peaceful and shaded atmosphere.

Archaeologists also discovered a sandstone lintel carved with the image of “Vishnu Reclining on Ananta,” a masterpiece of Khmer religious art. This suggests that the sanctuary was originally associated with Vaishnavism, a branch of Hinduism devoted to the worship of Vishnu, before Buddhism later became more influential in the region.Thailand’s Fine Arts Department has carried out restoration and stabilization work to preserve the sanctuary and prevent further deterioration. The surrounding landscape has also been improved for historical tourism. Today, Prasat Tamnak Sai stands as one of Sisaket’s important Khmer archaeological sites, reflecting the prosperity of ancient Khmer civilization in Southern Isan through its architecture, religious symbolism, and remarkable construction techniques.