During daylight hours, Gwanghwamun gate's portals are open to visitors entering and leaving the Gyeongbokgung Palace compound. Coming in or going out, make sure to look up as you pass through: the ceilings are vividly painted with mythical heavenly creatures.
Recently restored, this gate presides over the northern end of the main north-south axis in Seoul's historic center. It was originally built in 1395 as the main gate to Gyeongbok-gung Palace, the main seat of government for the newly established Joseon dynasty (1392-1910).
A couple of translation and pronunciation notes:
"Gwanghwamun" (Gwahng-hwah-moon) literally means "light spreading over the world gate," and "Gyeongbokgung" (Gyoung-bohk-goong) can be rendered as "The Palace of Shining Happiness."
To get here via subway:
Line #3--Use exit #5 of Gyeongbokgung Station.
Line #5--Use exit #2 of Ganghwamun Station