Northern metropolitans in the Far East

Interesting question. I don't think there were any ancient cities north of today's Beijing that came even close to its size and scale. Sang Kyung, (in China's Jilin province), the capital city of Balhae Kingdom, was said to emulate T'ang's Chang'an. The city was encircled by the outer wall which was fortified by the encircling inner wall. The palace was located within the inner wall.

Another city that I can think of is Karakorum, (in today's Republic of Mongolia, near the Orkhon River, SW of Ulaanbaatar). The area around Karakorum had been inhabited by nomadic Turkic tribes from the 1st century. A.D., but the city itself was not laid out until c.1220, when Genghis Khan, founder of the Mongol empire, established his residence there. As capital of the Mongols, Karakorum was visited (c.1247) by a papal mission under Giovanni Carpini. The city was abandoned (and later destroyed) after Kublai Khan, grandson of Genghis, transferred (1267) the Mongol capital to Khanbaliq (modern Beijing). The noted Lamaist monastery of Erdeni Dzu was built near Karakorum in 1586. The ruins of the ancient Mongol city were discovered in 1889 by N. M. Yadrinstev, a Russian explorer. (http://www.factmonster.com/ce6/sci/A0827074.html)

Another ancient city north of Beijing was Shangtu (or "Kaipingfu"). It was founded 8 years before Yanjing or Khanbalik (Beijing), by Kublai Khan. The following is some information on Shangtu:

http://www.ceg.com.cn/old/2004/04E-2Q/04e2-n7.htm

"Shangtu was the second most important political, economic, and cultural center of the Yuan Dynasty besides its Great Capital, which is today's Beijing ...

In 1256, a city was built and it was then called "Kaiping Fu". Eight years later, Beijing was built and called "Great Capital," and Kaipingfu became "Shangtu," which means "Upper Capital." Since then emperors of Yuan have been shuttling between the two capitals ...

Kaipingfu, Kublai's city, remained the empire's political center for ten years, and then Yanjing (today's Beijing) was built after the Mongolians conquered the Central Plains ...

The existence of two capitals catered for demands of the Mongolian aristocrats, who couldn��t bear the heat in the summer of Beijing. It also satisfied the military needs - the ruling Mongolians had to move the political center to the south after they conquered the Central Plains, but they also needed a northern center, which lied at the throat of the only entry from the northern plateau to the southern plains. 

...

Shangtu was constantly expanded in the following decades until it became the largest and most magnificent capital in the northern grassland in the Chinese history. 

Residents of the world's metropolis included the noble and the grassroots, Chinese and foreigners, Mongolians, Hans, Khitans, Uygurs, Koreans, Nepalese, and Italians, who live together in harmony. 

The prosperity of Shangtu lasted about 100 years until it was burned down in the farmers' uprise at the end of the Yuan Dynasty and almost destroyed totally in the following years."