These are the ruins of Ayutthaya (also known as Ayudhya) which was the former capital of Thailand (or Siam) until it was attacked and destroyed by the Burmese. Ayutthaya was named after the holy Indian city "Ayodhya". After the destruction of Ayutthaya, Bangkok was founded as the new capital of Siam. Being much further south, Bangkok was was easier to defend. Much of Bangkok's layout (Grand Palace complex, canals, roads, etc) were copied from Ayutthaya.