Wat Phra Si Rattana Mahathat is the old centre's most substantial temple ruin, standing almost opposite the railway station. The site has Khmer roots and later Ayutthaya additions, so brick stupas, prang forms, and broken halls sit together in one open compound.
It is a better place than Phra Prang Sam Yot for slowing down and looking at the architecture. The monkeys are less central here, and the ruins give Lopburi's history more weight.