Seoul, South Korea itself

 
This is one of the night views of Seoul, taken from the top of Namsan Mountain in downtown Seoul.

Seoul was designated as the capital of the Joseon Dynasty in 1392 AD, right after its founding, and has been the capital of Korea for over 600 years until it became the current South Korea.  

Going back even further, in 18 BC, it was the capital of the Baekje Kingdom when it was founded. 

This dynasty, which fell in 660 AD, moved its capital to another place in 475 AD, and the current Seoul was the capital of that ancient kingdom for 500 years.
Therefore, Seoul was the capital of Korea for about 1,100 years, and is the current capital. 

The population of Seoul Metropolitan is about 10 million, and when combined with its satellite cities, it is a huge city with a population of 20 million.