Seoul (or Hanyang/Hansung during the Joseon dynasty), had been the capital of the country of Korea for quite some time. During the Joseon dynasty and during the Japanese Occupation, it had always been the capital. read more
Extra fun: In the late 1940s, when North Korea was divided from South Korea, it's amusing to note that the North Korean constitution literally had it written that "Seoul is the capital of North Korea, and Pyongyang is the temporary capital". read more
Indeed, if war ever broke out with North Korea, experts estimate that North Korea could wipe out Seoul. read more