Wat Phra Si Sanphet was the holiest temple in Ayutthaya. The royal temple which was built in 1350 was part of the Grand Palace where some former kings of Ayutthaya lived. The three iconic pagodas contain the ashes of three great kings of Ayutthaya.
As it was a royal temple and used only for royal ceremonies, no Buddhist monks lived in the temple. The same rule applies to the current royal temple, Wat Pra Kaew, in Bangkok.