From the World Heritage inscription:
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.
Ayutthaya is very similar to Skuhothai, except it is a much busier city and the attractions are spread out over a much larger area, not confined to a single park. It is a northern suburb of Bangkok and can easily be reach by taxi or tour bus in 30-60 minutes depending on traffic. They day I went was New Year’s day and all the temples were packed with people.