Bagan became a central powerbase in the mid 9th century under King Anawratha, who unified Myanmar under Theravada Buddhism. It is estimated that as many as 13,000 temples and stupas once stood on this 42 square km. plain in central Myanmar. Some stupas even date back to the 11th and 12th century.
Read more about the thousands of temples in Bagan: A Forgotten Wonder of the World