From China Digital Space
帝都 (dì dū): imperial capital
This is a recent internet nickname for Beijing. Formerly, the phrase “imperial capital” was used in reference to the capitals of imperialist countries, but as countries have ceased to proclaim themselves empires, the phrase “imperial capital” has become synonymous with a nation's capital.
Beijing's nickname of “imperial capital” is often contrasted with an internet nickname for Shanghai, 魔都 (mó dū), which means “devil/monster capital.”