Nanjing took the lead with 500,000 people by 1400.
Located in the Yangtze River Delta, Nanjing was the capital of the Ming dynasty.
It was here in 1405 that the Yongle Emperor launched the first of several treasure voyages, sending fleets of the biggest wooden ships ever to travel as far as Africa (or by some accounts America).
Not long after, however, political struggles forced the emperor to move his capital to Beijing, and Nanjing declined.