Ayutthaya was the capital city of the glorious empire of Siam for more than 400 years, to be precise from 1351 to 1767. Then the Burmese razed the city. The capital of Siam moved first to Thonburi, a few years later to Bangkok.
Most of Ayutthaya's ruins can be found on the central 'island' but there is also a lot outside. Many of them have become integrated in today's city.