Wat Tum is located off the city island in the northeastern area at Tambon Wat Tum. The monastery is situated on the south bank of Khlong Wat Tum, a canal which had its mouth at the old Lopburi River and can be reached via Road No 309 linking Ayutthaya with Ang Thong. The temple is still in use by the Buddhist clergy and covers an area of approximately 15 rai.
Wat Tum lays south of Wat Chumphon, Wat Chang Yai and Wat Chang Noi, three temples related to warfare, where in its vicinities war elephants were trained and troops were gathered prior to go for battle. Some sources state that King Naresuan held the ceremony of "Drinking of the Water of Allegiance" at this temple, a very important ancient rite. (1)
The ceremony of "Drinking of the Water of Allegiance" was one of the most important Ayutthayan state ceremonies from the point of view of the upkeep of the established form of government. It was a ceremony derived from the Khmers which performed this kind of activity already in the 10th/11th century AD following engravings found on the pillars of a portico near the Phimeanakas at Angkor. The ceremony was held on a regular bases. During the Ratanakosin period, the rite took place twice yearly, on the third day of the waxing of the fifth month (Chaitra) and on the thirteenth day of the waning of the tenth month (Bhadrapada), in the Chapel Royal in Bangkok as in one temple in each seat of provincial government. The Ceremony of Drinking the Water of Allegiance in the Ayutthaya period was the same as what was followed in Bangkok from the reign of Rama I onwards. 
Horace Wales described the rite as follows: "The water is previously hallowed in the usual way by the monks, who recite mantras, the sacred "sincana" thread being stretched round the water vessels, while the Court Brahmans also dip into the water the State Sword and other royal weapons, this being a rite of contagious magic, by the power of which any official meditating treason would be destroyed. It is said that persons have not infrequently in the past died of cholera after drinking the hallowed water, a result which no doubt did much to strengthen the general belief in the efficacy of the magic. On the day of the ceremony a Brahman reads out the Oath [of Allegiance] and each official must drink the contents of a small cup, which he must drain to the last drop. Any appearance of difficulty in swallowing was in the old days, considered as equivalent to an admission of disloyalty. The ladies of the palace, as well as members of the royal family, drink the Water of Allegiance but, of course, with suitable privacy. Officials confined to their houses through illness am not excused from drinking the Water, but it in taken to their bedsides by royal pages or other officials." 
Simon de La Loubère mentions this Oath of Allegiance in his work "A new Historical Relation of the Kingdom of Siam" published in 1691: "The Form of the Oath of Fidelity consists in swallowing the water, over which the Talapoins do pronounce some Impreciations against him, who is to drink it, in case he fails in the Fidelity which he owes to his King." 
A brochure of the Fine Arts Department states that this temple was built prior the establishment of Ayutthaya in 1351. 
The monastery must have been badly damaged during the fall of Ayutthaya in 1767. The site was not looked after until in the Early Ratanakosin period during the reign of King Rama I (r.1782-1809), when Siam recovered slowly from its wounds of the Burmese war, the temple was restored and occupied by monks. During the reign of King Mongkut (r.1851-1868), the monastery received royal patronage and since that time the Royal standard is flown.
In situ are multiple monastic structures from recent times. The old ordination hall or ubosot has been built in Early Ayutthaya style and has been redone in recent times. The ubosot houses a special Buddha image called “Luang Phor Thong Suk” of unknown origin. It is a bronze crowned and bejewelled image of a seated Buddha in the gesture of subduing Mara, measuring 87 cm in width and 1.5 m in height. The cranial part of the head can be lifted. The head is hollow and contains water, formed in a natural way in the image's head. Since old times, Thai people come from far away to take a sip from the holy water, believed to have healing capacities.
(1) Likely the ceremony held here was not the "Drinking of the Water of Allegiance", but the pre-battle rite of "Cutting the wood which corresponds with the name of the enemy". See for an explanation Wat Chang Yai - Footnote 6.
 Siamese state ceremonies: their history and function - Horace Geoffrey Quaritch Wales (1992) - Page 193-196.  A new Historical Relation of the Kingdom of Siam - Simon de La Loubère (1691) - Page 81.  Ayutthaya Historical Park - Fine Arts Department (2003).
Text & photographs by Tricky Vandenberg - September 2009