It is commonly known as Namdaemun (南大門), meaning the Great South Gate, as it served as the southern main gate of the Seoul city wall.

Construction began in 1395 (the 4th year of King Taejo's reign) and was completed in 1398 (the 7th year of King Taejo's reign), with a reconstruction carried out in 1447 (the 29th year of King Sejong's reign).

The gate stands atop a massive stone foundation with an arched gateway (hongnyemun) at its center, and is the largest existing city gate structure in Korea.

It was designated as National Treasure No. 1 on December 20, 1962.

As the oldest surviving wooden structure in Seoul, a fire that broke out on February 10, 2008 destroyed the second-floor gate pavilion and partially burned the first-floor gate pavilion, which has since been restored.

Address : 40 Sejong-daero, Jung-gu, Seoul (Namdaemun-ro 4-ga)

Inquiries : 042-481-4650

Hours : February~May / September~October 09:00~18:00

              June~August 09:00~18:30

              November~January 09:00~17:30

Source

http://www.junggu.seoul.kr/tour/