Wat Pom Yai, or the Monastery of the Large Fortress, is an active monastery located off the city island in the western area of Ayutthaya in the Ban Pom Sub-district. The monastery borrowed its name from the old site of Wat Pom Yai, dating to the Ayutthaya period. The site can be accessed via road No 3412.
The old site of Wat Pom Yai stood on the connection canal leading to the Chao Phraya River at Hua Taphan via Khlong Maha Phram. Wat Phra Ngam was situated north, and Wat Pom Noi south. Wat Phukhao Thong was on the opposite side of the connection canal. Between Wat Pom Yai and Wat Pom Noi was a canal called Khlong Wat Pom (some maps have Khlong Worachet), which had a junction with the connection canal leading to the Chao Phraya River. Opposite the mouth of this khlong was another canal leading to Wat Phukhao Thong called Khlong Yai.
In the Ayutthaya era, large wooden forts were in the four directions. The position of one of these forts can be seen on a map by Jacques Nicolas Bellin (1703-1772 CE): Carte du Cours Du Menam Depuis Siam Jusqu'a La Mer Levee sur los Lieux par un Ingenieur Francois and published as plate no. 5 in volume 9 of the 1752 French edition of Abbe Provost's L'Histoire Generale des Voyages.
Nothing remains above ground level of the old Wat Pom Yai. The site was covered with vegetation and somewhat marshy. On the site's eastern side (along Khlong Wat Pom), there was an earthen wall, which may contain remnants of the monastery's outer wall, although this remains a guess.
The site is heavily prone to inundation. For this reason, a new temple with an identical name was established in 2000 CE and, based on the Temple Registration System of the National Office of Buddhism, received its Wisung Kham Sima in 2014 CE.
