Founded c. 1350, Ayutthaya became the second Siamese capital after Sukhothai. It was destroyed by the Burmese in the 18th century. Its remains, characterized by the prang (reliquary towers) and gigantic monasteries, give an idea of its past splendour. Read more here http://whc.unesco.org/en/list/576 and here http://en.wikipedia.org/wiki/Ayutthaya_Historical_Park
The pictures weren't all taken there, but I can't remember the names of all the places we visited on our trip in January 2009.