Located in the grounds of the old Ayutthura city is the temple of Wat Phra Si Sanphet. It includes the tombs of three Kings.
Wat Phra Si Sanphet was the holiest temple on the site of the old Royal Palace in the ancient capital. It housed a gold Buddah 16 meters high (over 300 kg of gold) and also included three Chedi’s where the ashes of three kings are buried. The city was destroyed by the Burmese in 1767 (and the gold taken).
And here’s more on the history (according to Wikipedia) if you want to know more:
In 1350 King Ramathibodi I, ordered the construction of a royal palace in the same area that Wat Pra Si Sanphet stands today. The palace was completed and King Ramathibodi established Ayutthaya as his capital. The palace contained three wooden prasats named “Phaithun Maha Prasat”, “Phaichayon Maha Prasat”, and “Aisawan Maha Prasat”. However, in 1448 King Borommatrailokanat built a new palace to the north and converted the old palace grounds into a holy site. His son, King Ramathibodi II had two Stupa, which in Thailand are known as Chedis, built in 1492 where the ashes of his father, King Borom Trailokanath, and his brother, King Borommaracha III were buried.
In 1499 a viharn, or hall of worship, called “Vihara Luang” (Royal Chapel) was built on the palace grounds. King Ramathibodi II gave orders for a gigantic image of Buddha to be cast, and installed in Wat Si Sanphet. This image of Buddha was 16 meters high, covered in gold, and the pedestal was 8 meters in length. The core of the statue was made of bronze and weighed approximately 64 tons. The surface was covered with approximately 343 kilograms of gold. This statue, called “Phra Si Sanphetdayan”, was the main object of veneration within the royal chapel. Another Chedi was built under King Borommaracha IV in 1592.
The city of Autthaya including the temple compounds were completely destroyed in the Burmese invasion in 1767, with the exception of the three Chedis that can be seen today.
Wikipedia – Wat Phra Si Sanphet https://en.wikipedia.org/wiki/Wat_Phra_Si_Sanphet Accessed 19 Sept 2015