Ayutthaya city is the capital of Ayutthaya province in Thailand. The city was founded in 1350 by King U-Thong, who came here to escape a smallpox outbreak in Lop Buri, and proclaimed it the capital of his kingdom, often referred to as the Ayutthaya kingdom or Siam. Ayutthaya was named after the city of Ayodhya in India, the birthplace of Rama in the Ramayana (Thai, Ramakien). In 1767 the city was destroyed by the Burmese army, resulting in the collapse of the kingdom, and the ruins of the old city now form the Ayutthaya historical park, which is recognized internationally as a UNESCO World Heritage Site. The city was refounded a few kilometers to the east.
It is estimated that Ayutthaya around 1600 had a population of about 300,000, and even 1,000,000 around 1700, making it one of the world's largest cities at the time.
References sources : wikipedia