Wat Prasat, or the Monastery of the Castle, was located in the southern part of Ayutthaya’s city island in the Pratu Chai Sub-district. The restored ruin stands on the premises of the Phra Nakhon Si Ayutthaya Hospital near the intersection of U Thong Road and Si Sanphet Road.
The monastery was situated between Khlong Pratu Chai and Khlong Chakrai Noi. It stood near the beginning of one of the most important roads of the city at that time, being the Maha Rathaya Road, also called Thanon Luang, a smooth paved laterite road, about 12 meters wide, which ran through the centre of the capital and linked the Royal Palace with the southern city gate. (1)
Wat Prasat was located west of the temple cluster of Wat Khok Khemi, Wat Khok Khema and Wat Khok Yai Mi.
Only a sizeable octagonal base of a chedi in the classic Ayutthaya style and its toppled part remains.
Historical data about the monastery and its construction are unknown.
Maps
I believe Wat Prasat shows on Kaempfer's sketch. The monastery (or the chedi) stood on the west bank of Khlong Chakrai Noi near the Chakrai Noi Gate. Engelbert Kaempfer (1651-1716 CE) was a medical doctor working for the Dutch VOC (Verenigde Oost-Indische Compagnie) who surveyed Ayutthaya in June 1690 CE.
Wat Prasat also appears on Bellin’s map ‘Plan De La Ville De Siam’. Jacques-Nicolas Bellin (1703-1772 CE) was one of the most important cartographers of the 18th century. He was a hydrographer and 'ingénieur hydrographe' at the French 'Dépôt des cartes et plans de la Marine'. The map ‘Plan De La Ville De Siam’ of the French cartographer Jacques Nicolas Bellin (1703-1772 CE), based on a Jesuit survey in 1687 CE was published as plate No. 4 in volume 9 of the 1752 CE French edition of Abbé Antoine François Prévost's l'Histoire Générale des Voyages. The map shows Wat Prasat between Khlong Pratu Chai and Khlong Chakrai Noi at the beginning of the ‘Rue Napetat’, Maha Rathaya Road or Royal Road. Wat Suan Luang Khangkhao stood northeast opposite the Chakrai Noi Canal. Bellin shows also the old and new French Lodge and the Hotel of the Ambassadors of France.
On a 19th-century map by an unknown surveyor, the monastery stands on the west bank of Khlong Chakrai Noi, east of Wat Khok Yai Mi and South of Wat Krabueang Klaep (Wat Borom Phuttharam). Wat Suan Luang Khangkhao was in the northeast on the opposite bank of the canal. The temple is indicated with a single chedi.
The site is indicated on Phraya Boran Ratchathanin's (PBR) map drafted in 1926 CE. Wat Prasat stood on the west side of the main road running along with Khlong Chakrai Noi and north of what PBR calls Thanon Rong Krai (2). Wat Thong Pak Mo stood in the northwest. Phraya Boran (1871-1936 CE) was the Superintendent Commissioner of Monthon Ayutthaya from 1925 till 1929 CE.
The restored ruin of Wat Prasat is in geographical coordinates: 14° 20' 37.77" N, 100° 33' 39.26" E.
Footnotes:
(1) This main road was used for the royal ordination procession, royal funeral procession, and other funerals. The processions started from the Victory Gate on the south of the city. [1]
(2) Clown featherback Barn Road. The clown featherback (Chitala Ornata) is a nocturnal tropical fish with a long, knife-like body. This fish is native to freshwater habitats in Thailand.
