The principal monument of this temple is the prang, which was derived from a Khmer tower. But a stupa, like the one you have just seen at Wat Yai Chaiya Mongkhon, had its originin India. However, both a stupa and a prang represent the Lord Buddha.
It’s a matter of the preference of the temple. The construction of a prang as the principal monument was popular from the first reign to the eighth reign of Ayutthaya. From the ninth reign onward the construction of a stupa came into fashion. As this temple was built by the seventh reign of Ayutthaya, Chao Sam Phraya, a prang was built. According to a chronicle this temple was built on the cremation site of the two elder brothers of Chao Sam Phraya who were both killed while fighting each other for the throne. However, some scholars suggest that this temple was built on the cremation site of King Nakharinracha or Nakhon In, their farther