Deoksugung / Gyeongbokgung
Five grand palaces were built by Korean royalty in Seoul and stood until being largely destroyed by the Japanese at the beginning of their occupation in the early 20th century.
Deoksugung was initially the home of Wolsandaegun (1454-1488), the older brother of King Seongjong. Gwanghaegun (1575-1641) made it an official palace in 1611.
Gyeongbokgung (Kyungbok Palace) was the main royal palace of the Joseon Dyasty and was originally built in 1395.
The Namdaemun gate was built in 1398 as one of three major gateways through Seoul's city walls. Those city walls were demolished in the early 20th century.
If you've got it, and you can... you should.