Wat Sing, or the Monastery of the Lion, was located on Ayutthaya's city island in the city's northwestern area in the Pratu Chai Sub-district. The temple was situated between Khlong Fang (1) and Khlong Chang Maha Chai (2). Wat Khok (defunct) was northeast, while Wat Sop Sawan (defunct) and Wat Noi (defunct) were south.
There are no traces anymore of the former monastery. A Thai Army camp (old cantonment) has been installed on the site. I classified the temple as defunct.
Historical data about the monastery and its construction are unknown.
In the late Ayutthaya period, a fresh market was held before the monastery called Lion's Head Market. [1]
Wat Sing on the maps:
Wat Sing shows on a 19th-century map by an unknown surveyor. The monastery is situated west of the old road running east of the Rear Palace grounds in a bow to the Pak Tho Canal Gate. Wat Noi was south, Wat Sop Sawan southwest, Wat Khok north and Wat Chettharam east. The map indicates the existence of a chedi.
The monastery is in a near-identical position on Phraya Boran Rachathanin's map drafted in 1926 CE. Phraya Boran (1871-1936 CE) was the Superintendent Commissioner of Monthon Ayutthaya from 1925 till 1929 CE but occupied important functions since 1896 CE in Monthon Ayutthaya. Wat Sing is south of Khlong Fang and between the U Thong Road and the old road running east of the Rear Palace grounds in a bow to the Pak Tho Canal Gate. Wat Sop Sawan was slightly southwest and Wat Khok Chang slightly southeast both temples are near Khlong Chang Maha Chai and separated by the old road.
Wat Sing shows on a 2007 GIS Fine Arts Department map in geographical coordinates 14° 21' 21.10" N, 100° 32' 53.21" E, but this is, in my opinion, unlikely as this location is on top of the former Ayutthaya city walls or even outside. The 19th-century map, as well as PBR's map, indicate the temple within the city walls.
Wat Sing was likely in approximative geographical coordinates: 14° 21' 19.10" N, 100° 32' 57.81" E.
Footnotes:
(1) Khlong Fang, or the Rice Straw Canal, is a defunct canal situated before in the Pratu Chai Sub-district. The east-west running canal had its mouth at the old Lopburi River near Hua Laem and linked up with Khlong Pak Tho. The canal pierced the fortified city wall at the Khlong Fang Gate, a large water gate between Wat Sing and the Satkop Fortress. The canal has been filled up somewhere after the fall of Ayutthaya (1767 CE), and no traces of the waterway are left today, except for a part of the moat of Wat Worachetharam. Khlong Fang extended as a small ditch into the Grand Palace grounds feeding Sra Kaeo (Crystal Pond).
(2) Khlong Chang Maha Chai, or the Great Victory Granary Canal, is a defunct waterway on Ayutthaya's city island in Pratu Chai Sub-district. The east-west running canal had its mouth at the old Lopburi River in front of Wat Thamma and linked up with Khlong Pak Tho. The canal passed the fortified city wall at the Chang Maha Chai Gate, a large water gate between Wat Suan Luang and Wat Sop Sawan. The canal has been filled up somewhere after the fall of Ayutthaya (1767 CE), and no traces of the waterway are left today.
References:
[1] Baker, Chris (2011). Before Ayutthaya Fell: Economic Life in an Industrious Society. Markets and Production in the City of Ayutthaya before 1767: Translation and Analysis of Part of the Description of Ayutthaya. Journal of the Siam Society. Vol. 99. p. 65.