From 1350 to 1767, Ayutthaya served as Siam's capital. Then it was sacked by the Burmese, its buildings were stripped of gold by the kilos, and its people were enslaved. When the kingdom rose again it did so in Rattankosin (now Bangkok).
At least, that's my understanding as a farang-come-lately and history geek with a touch of ADHD, so I could be off on some details. But I do know I rode a minibus an hour today to visit the ruins at Ayutthaya. Here are some pics and notes.