Karakorum was an ancient palace and “capital city” of the Mongol Empire in the 13th century, although for only about 30 years. Today its ruins lie in the southeastern corner of the Arhangai Province of Mongolia or, to be more specific, in the upper part of the World Heritage Site entitled Orkhon Valley Cultural Landscape

In 1585 Abadai Khan of the Khalkha built a Tibetan Buddhist monastery called Erdene Zuu (Erdeni Juu) on the site. Various construction materials were taken from the ruin to build this monastery.

from wikipedia
http://en.wikipedia.org/wiki/Karakorum_%28Mongolia%29