Spread over 42 km2 on the left side of the Ayeyarwady river, Bagan is one of the most famous archeological sites of South East Asia. It was built in only two centuries, yet it contains 2217 sanctuaries still standing and probably had about 5000 during its golden age. Bagan was the capital of Burma for 230 years (11th to 13th century). The city then included many wooden houses and palaces which decayed leaving us with this extraordinary scenery of a wide plain dotted with hundreds of brick pagodas and temples. Bagan's history started with King Anawrahta's ascension to throne in 1044 (he's the one who introduced Theravada Buddhism to Burma) and ended with the Mongol invasion of Emperor Kublai Khan in 1287. Anawrahta unified the country by conquering the Môn kingdom of Thaton in 1057. He brought back the 30000 inhabitants of Thaton as slaves to Bagan. Môn architects were to have a considerable influence on the earliest constructions (especially between 1057 and 1120).
Ananda, constructed under kings Anawarhta and Kyanzittha, is a good example of Môn style used in Bagan. It has a squarish central structure, each side measuring 53m in length, with a height of 10m. Four entrance corridors give the base structure the shape of a cross. On top of this structure, 6 terraces are piled up forming a pyramid with a hti on its top.
Another key sanctuary of Bagan is Shwedigon, which is today's main pilgrimage center in the area. Shwezigon too was built under kings Anawarhta and Kyanzittha. However, its style is truly different: it's the first large monument built in a typical Burmese style, showing a real departure from Môn architecture. It was used as a model for many later sanctuaries. It's a golden stupa built on 5 terraces (3 square ones and 2 circular ones), each terrace symbolizing a stage in Nirvana.
Another temple worth mentioning is Dhammayangyi. It was built on a plan similar to Ananda (even though it was built in a later period), but it's size is even more impressive. It was built by King Narathu (1169-1174), which was famous for his monstrous bloodthirstiness. He first got hold of the royal throne by murdering his own father Alaungsithu and his elder brother Minthinsaw. He then had one of his wives, an Indian princess, killed because she displeased him. After displaying so much brutal ferocity, he decided it was time to expiate and had a temple built. Yet he ordered the hands to be cut of all the builders who didn't adjust the bricks well enough and finally had the architect executed so that nobody could ever build a similar temple again. Narathu finally died in the hands of a group of Indians sent by the father of his late wife. There is much much more to say about Bagan. About the temples and stupas themselves or about the 1975 earthquake, the resettling of the inhabitants to New Bagan by the government etc… but I'll try to keep my info text to a reasonable length: those who want to know more will easily find more detailed sources.