Debate remains over whether King Prasat Thong (reign 1629-1656 CE) or King Narai (reign 1656-1688 CE) ordered its construction.
The Office of Natural Resources and Environmental Policy and Planning (ONEP) heritage database and Thepsatri Rajabhat University both suggest that King Prasat Thong built Tamnak Sa Yo around the same time as Phra Tamnak Than Kasem. The evidence cited is the resemblance of the surviving stone-and-mortar foundations to those of other late Ayutthaya royal structures in the area. [1]
The same ONEP record reports another tradition: in the Kham Hai Kan Khun Khlon (คำให้การขุนโขลน) (Testimony of Khun Khlon on the Buddha's Foot), the site is called “Tamnak Phra Narai Pen Thai Sa Yo”—a name associating it with King Narai. Under this interpretation, King Narai may have ordered the pavilion built, perhaps modelling it on Tamnak Than Kasem to accommodate royal travel and royal conveyances. [2]
King Narai ordered the construction of a road from Lopburi to the Phrabat. This route likely followed the present-day Phahonyothin Road (Highway 1) between Lopburi and Saraburi, linking the modern-day provinces. The road enabled the king and his retinue to travel with ceremonial processions, supported by staging pavilions, rest stops, and royal water systems uncovered during archaeological surveys. Phra Tamnak Sa Yo is not along King Narai’s route, but he ordered improvements to Thanon Farang Song Klong so the royal pavilion could have been erected there.
“Now the King ordered [men] to improve and beat flat the land route coming from the Footprint of the Holy Buddha by way of the rear of Peacock Springs and Stone Landing to make a crown road all the way to the Municipality of Lopburi, and ordered [men] to improve the road to the Arrow Dip Lake, the road to Crystal Pool, and the road to the boat landing in the vicinity of the Holy Habitation of the Lords' Pleasure Landing.” [3]
Tamnak Sa Yo is at geographical coordinates 14° 42' 27.44" N, 100° 47' 5.41" E.
References:
[1] Thepsatri Rajabhat University Library. (https://library.tru.ac.th).
[2] Office of Natural Resources and Environmental Policy and Planning (ONEP) heritage database (https://culturalenvi.onep.go.th).
[3] Cushman, Richard D. & Wyatt, David K. (2006). The Royal Chronicles of Ayutthaya. Bangkok: The Siam Society. p 308.