The site Kublai Khan chose was not empty. Zhongdu, the Central Capital of the Jurchen Jin dynasty, had stood nearby before Genghis Khan destroyed it in 1215. But Kublai wanted something new. He appointed Liu Bingzhong as chief architect, with Liu's student Guo Shoujing and the Muslim engineer Ikhtiyar al-Din assisting. Construction of the walls began in 1264, and the main imperial palace followed from 1274. The design drew on the Confucian classic Rites of Zhou, following principles that prescribed nine vertical and horizontal axes, palaces in front with markets behind, ancestral worship to the left and divine worship to the right. When Kublai proclaimed the Yuan dynasty in 1271, he named the city Dadu -- Grand Capital -- in Chinese, while diplomatic letters in Mongolian used Khanbaliq. The construction would not be fully completed until 1293, and Kublai's former seat at Shangdu became the summer capital that European writers would romanticize as Xanadu.

What set Khanbaliq apart from other medieval capitals was its radical religious diversity. The Great Khans practiced a policy of religious tolerance that turned the city into a spiritual crossroads unlike anything else in the thirteenth-century world. Rabbis maintained synagogues. Taoist sects operated temples. Mongol shamans performed their rituals alongside Hindu religious groups. Buddhists and Muslims were common, as were Christians from the Church of the East and Catholics who had traveled overland from Rome. Confucians and Taoists were especially well-regarded by the Mongol nobility, with some of the Mongols' most trusted advisors drawn from their ranks. The city printed paper money -- fiat currency called jiaochao -- from an Imperial Mint that may have been operating at nearby Yanjing even before the capital was formally established. In commerce and in faith, Khanbaliq was a place where the world converged.

When European travelers reached China by sea in the sixteenth century, arriving via Malacca and the Philippines, they did not realize they had found the same country Marco Polo had described. The name Cathay, drawn from Polo's accounts, seemed to describe a different land entirely, and Cambaluc a different city from the one the southern Chinese called Pekin. It took the Jesuit Matteo Ricci's visit to Beijing in 1598 to settle the question. There, he encountered Central Asian visitors who confirmed that the city they stood in was indeed the legendary Cambaluc. Ricci's published journals announced to Europe that Cathay was China and Cambaluc was Beijing. Even so, European mapmakers continued to show a separate Cathay with its own Cambaluc somewhere in northeastern Asia for much of the seventeenth century, unable to let go of the idea that such a place must exist independently of the known world.

When the Ming dynasty overthrew the Yuan in the fourteenth century, the new rulers renamed the city Beiping, meaning Pacified North. The Prince of Yan later shortened its northern boundaries, added a walled southern district, extended Taiye Lake to create the present-day Nanhai, and raised Wansui Hill over Yuan ruins. Upon completing the Forbidden City, he declared the capital Beijing -- Northern Capital -- and with one brief interruption, the name has held. But Khanbaliq has not entirely vanished. Ruins of the Yuan-era walls survive as the Tucheng, or earth wall, preserved in Tucheng Park along with modern statues. Several stations on Beijing Subway Lines 10 and 13 are named after the gates of Dadu, an echo of a Mongol city encoded in the daily commute of millions. The last Yuan emperor, Toghon Temur, composed a lament for the loss of Khanbaliq that is recorded in Mongolian chronicles. In Persian and Turkic languages, Khanbaliq remained the standard name for Beijing for centuries afterward.