Changdeokgung Palace,

was built in 1405, is one of UNESCO World Cultural Heritages in Korea. There are many beautifully Korean traditional architectures and you can travel back into the past of Korea without a time machine. Especially, I strongly recommend visiting the Sceret Garden filled with trees and architectures. If anyone plan to trip to Seoul, don't miss this place!
*fyi : eng.cdg.go.kr/main/main.htm