Ayutthaya

Ayutthaya was the capital of the kingdom of Siam from 1350 until 1767 when a Burmese army destroyed the city. The ruins of the temples are recogized as a UNESCO World Heritage Site.

The now bare stones of temples were once covered in gold. They gold was taken by the Burmese when the city was looted and it is said by thai people until today that they used the gold for the Shwedagon pagoda in Rangoon.

Ayutthaya can easily be reach on a 2h train ride into the north of Bangkok. There are also frequent buses from Bangkok to Ayatthuya. As the temples are fairly far apart it is easiest to hire a bicycle when you arrived in Ayatthuya.

train fares and distances from Bangkok
Wikitravel: Ayutthaya

Wikitravel says:

It is advised to rent a bicycle for around 40-50 Baht per day. You should get a copy of a map for free at the shop that rents you the bicycle. If you are physically larger than most Thais, be warned that the larger bicycles are not necessarily well maintained, so be sure that they work properly (seats well attached, handlebars don’t slip in relation to front wheel direction) before you leave.
Alternatively, you can hop around town by tuk-tuk. Ayutthaya’s tuk-tuks are larger than the Bangkok variety and you can easily squeeze in four or more on the two songthaew-style facing benches. Only “official” tuk-tuk drivers or tourist “helpers” can pick up passengers from the train station. You can verify their status by looking for their photos/name on a “Tourist Officials” board displayed at the southern end of the platform. These people are required to charge/work for fixed charges, usually quoting 300baht/hour, but this can usually be bargained to a slightly lower price (eg. 1000baht for 4hrs). The drivers are very helpful, and will bring you to most of the popular destinations.

Wikipedia says:
Ayutthaya (full name Phra Nakhon Si Ayutthaya, Thai: พระนครศรีอยุธยา, also spelled "Ayudhya") city is the capital of Ayutthaya province in Thailand. The city was founded in 1350 by King U-Thong, who came here to escape a smallpox outbreak in Lop Buri, and proclaimed it the capital of his kingdom, often referred to as the Ayutthaya kingdom or Siam. Ayutthaya was named after the city of Ayodhya in India, the birthplace of Rama in the Ramayana (Thai, Ramakien). In 1767 the city was destroyed by the Burmese army, and the ruins of the old city now form the Ayutthaya historical park, which is recognized internationally as a UNESCO World Heritage Site. The city was refounded a few kilometers to the east. It is estimated that Ayutthaya around ca. 1600 had a population of ca. 300,000, and even 1,000,000 around 1700, making it one of the world's largest cities. ... read more