Map of medieval Chang'an (China) in the medieval era (the peak of the Tang Dynasty, around 750 AD):
Chang'an was by far the largest city in the world at the time, boasting between 1 and 2 million residents. This was probably twice as large as any other city in the world at the time. This map also illustrates some of the religious diversity in China at the time, as the Tang were quite open to the rest of the world relative to other Chinese dynasties.
It is striking that, even though a city of this size was a massive anomaly in the medieval world, we are projected to have 546 1 million-plus cities by 2025… up from 75 in 1950 (http://blogs.ft.com/beyond-brics/2011/10/21/chart-of-the-week-the-million-plus-cities/)!