Not far from Bangkok—about an hour by car—is the former capital of the Kingdom of Siam, the city of Ayutthaya. Now the UNESCO recognized Ayutthaya Historical Park, Ayutthaya played a major role in Thai history. Ayutthaya was the seat of 5 dynasties and 33 kings, and also was involved in over 70 wars (mostly with the Khmer of Cambodia and the Burmese of Myanmar). Ayutthaya was built in 1350 and named after the Indian birthplace of Rama. It remained the capital for 417 years until 1767 when the Burmese sacked the city. The Bangkok Empire was created by those who fled the destruction of Ayutthaya.
The ruins of Ayutthaya are spread out around the modern city, though it’s fairly easy to see the main temples such as Wat Phra Si Sanphet, Wat Mongkhon Bophit, Wat Na Phra Meru, Wat Thammikarat, Wat Ratburana and Wat Phra Mahathat in the city center by walking around (or taking one of the colorful tuk tuks!). I visited soon after the major flooding of 2011, so there was a lot of debris around still and some of the temples were closed to visitors. You can easily make out the high water marks on the temples in the photos.