Gwanghwamun is the ceremonial heart of Seoul — the square with the King Sejong and Admiral Yi statues, Gyeongbokgung Palace just up the avenue, the Sejong Center and the Cheonggyecheon stream all within a short walk. Store your bag and take it all in light.
Coin lockers at Gwanghwamun Station
Gwanghwamun Station (Line 5) has T-Locker coin lockers at 5 spots, about 118 in total — a large bank. The two biggest are on B1 toward Exit 2 (about 46 and 29, by the fare gates), with three more on the B2 concourse toward Exits 1 and 8. See each spot and the live count by size on the Gwanghwamun Station page.
Prices are the same across Seoul — about ₩2,200 (small) to ₩4,400 (large) for four hours, then hourly. English screen, pay by card, no ID.