Harhorin, or Kharkhorum or Karakorum, or whatever, there isn't a correct Western spelling for this ancient Mongolian word, became the capital of Mongolia when Chinggis Khan decided to move the capital from the Khentii aimag. Kharkhorin only served as capital of the Mongolian empire for 40 years, because after Khan's death the capital was moved to what is now Beijing. After the collapse of the Mongolian empire the capital was changed to Ulaanbaatar and Kharkhorin was left to decay.
In the 16th century Buddhist monks used the remains of the city to construct the Erdene Zuu Khiid monastery, and was the countries largest for centuries, with nearly 100 temples and 1000 monks in residence in its heydey. However, like Kharkhorin, Erdene Zuu Khiid was eventually abandoned completely, and most of the temple complex was destroyed in Stalinist purges in the 1930s, which destroyed all but three of the temples.
The ugly Soviet built modern city center holds little of interest for the visitor, but the monastery, history and surroundings make this city worth visiting.