Bangkok wasn't always the most important city of the Thai region. Ayutthaya, about 80km north from there, was the capital of the former Kingdom of Siam from 1350 to 1767. It was a large and crucial trading point as it lies between China, India and the Malay Archipelago. For a period of time it was the largest city in the world until the Burmese raided Siam and almost entirely burned Ayutthaya to the ground. All the libraries, houses and some of the temples were destroyed, which is a big pity because the kingdom was very prosperous and advanced at this time.
Today a few ruins still remain on site. By "a few" a mean a complex of about 9 temples, all open for public sightseeing.
