Theravada Buddhism

The area around Bagan has been settled since the second century AD, and in 849 King Pyinbya built the walls of the city, the remains of which can still be seen today. The "golden age" of Bagan, when most of the temples were built, began with ascension to the throne of King Anawrahta in 1044.
The temple building began after King Anawrahta conquered the Lower Burma Mon kingdom of Thaton in 1057. The king had converted to Theravada Buddhism and set about spreading it throughout the land. The practice was carried on by most of his successors, and by the time Kublai Khan's armies overran Bagan in 1287, there were some 13,000 temples, pagodas and other religious structures spread across the plain