Buddhism was declared Mongolia's national religion in the 13th century and has been widespread since the 1600s. By 1900, there were 583 monasteries and temples and a trip around some of them can be considered the perfect introduction to Tibetan Buddhism. Among the wonderful monasteries on this tour is Erdene Zuu, built in the 1580s on the site of the ancient imperial capital. One of the oldest monasteries in Mongolia, in its prime there were 10,000 lamas in residence and nearly 60 temples. By the end of the Stalinist purges in the 1930s, however, all but 4 of these temples had been destroyed.