Phra Nakhon Khiri, commonly known as Phra Nakhon Fortress, is situated on the southeastern side of the ancient city of Ayutthaya, in the Horatanachai Subdistrict, Phra Nakhon Si Ayutthaya District, in Phra Nakhon Si Ayutthaya Province. It is located at Bang Kaja, where the three rivers converge: the Chao Phraya River, the Pa Sak River, and the Lopburi River. Phra Nakhon Fortress is the most significant of the 29 fortifications that once stood in this area, with only two remaining today. It served a crucial role in defending against enemy invasions.