Zhongdu served as the Jin capital for more than 60 years, until the onslaught of the Mongols in 1214. The Mongols, a tribal nomadic people from the Mongolian Plateau and southern Siberia, had assisted the Jurchen in the war against the Khitans, but were not given the promised compensation. In 1211, the Mongols led by Genghis Khan took revenge against the Jin by invading northern China. By 1213, he had controlled most of Jin territory north of the Yellow River with the exception of the capital Zhongdu. In March 1214, he set up headquarters in Zhongdu's northern suburbs and with brother Hasar and three eldest sons, Jochi, Chagatai and Ögedei, began to besiege the city. Though the Jin court was weakened by a palace coup, the city was protected by three layers of moats and 900 towers. When disease broke out within the Mongol ranks, Genghis Khan sent Muslim envoy Ja'far into the city to negotiate, and the Jin court agreed to a peace treaty by ceding territory and accepting vassal status. Among Genghis Khan's demands was marriage to a Jurchen princess. The Qicheng Princess, daughter of Wanyan Yongji, was designated for the Mongol chieftain. She along with 100 guards, 500 boys and girl servants, 3,000 bolts of cloth, and 3,000 horses were sent to the Mongol camp. The Qicheng Princess became one of the four main wives of Genghis Khan, who lifted the siege and withdrew north of the Juyong Pass.