Coruscant

Coruscant was a massive ecumenopolis, meaning that the entire planet was one massive city. The capital of every major civilization that has controlled the galaxy, it was first established as the Capital of the Galactic Republic at about 25,000 BBY. Over the course of those twenty five millennia, the planet was completely covered by buildings. The population of Coruscant is estimated to be about one trillion, however this excludes transients and undocumented citizens, causing the exact population at any given moment to be perhaps tripled.

History
Coruscant has served as the capital of not only the original Galactic Republic, but additionally the Galactic Empire, the New Republic, the Yuuzhan Vong, before finally being brought back into the hands of the Galactic Alliance.

Geography
The planet itself is one giant city, with almost none of the original planet remaining. When the planet was finally covered entirely by buildings and there was no more space remaining, architects simply built on top of previously existing buildings. It is estimated that there are over five thousand levels on Coruscant, with the lower levels receiving almost no sunlight and being forced to rely on artificial lighting. Large pipes carried water across the planet, and huge parts of Coruscant were covered in abandoned industrial facilities. A few important locations include the Galactic Senate Building, the Jedi Temple, and the Works.