It stood for over 600 years but was destroyed by flames in one night.

In the heart of Seoul, the Namdaenum or South gate stood. The 14th century structure is a Korean landmark. According to Yonhap News:
The landmark, officially called Sungnyemun, or “gate of revering decorum,” was the southern gate of the walls that surrounded Seoul during the Joseon Dynasty (1392-1910). The 600-year-old gate survived a series of Korea’s tribulations from the Japanese invasions in the 16th century to the 1950-53 Korean War. Even in modern times when repair efforts on artifacts became commonplace, the historic gate kept most of its original form after it underwent post-war renovation in 1962.

It withstood the tribulations of history, so what could be the cause of this disaster. The investigation is ongoing but police suspect either an electric fault or arson caused the fire. It’s especially tragic if the latter is the case. According to Yonhap News:
The two-story wooden structure at the heart of Seoul collapsed about five hours after it caught fire late Sunday. The blaze appeared to be under control at one point, but it flared up again a few minutes later, spreading quickly as firefighters tried to salvage the iconic landmark.
The incident came as a shock for many Koreans, destroying their National Treasure No. 1 at the onset of the Lunar New Year.
Police suspected either an electric fault or arson caused the fire. But no tangible clues have emerged as they questioned witnesses and studied footage from surveillance cameras that had been installed at the historic gate, said Kim Young-su, head of Namdaemun Police Station whose jurisdiction covers the gate, said.

The tragic news is still developing.
Centuries-old gate collapses in fire (Yonhap)
Centuries-old gate collapses in fire (The Hankyoreh/Yonhap)
Fire Destroys Historic Seoul Landmark (Chosunilbo)
Seoul’s historic gate destroyed (CNN)
Rebuilding Discussed for Burned Sungnyemun (KBS)
I’ll post updates below…
By Shay on February 11, 2008 at 3:18 am
Update: Arson suspected in Namdaemun gate fire in Seoul (Wikinews)
By Shay on February 11, 2008 at 7:18 pm
Update: Police Arrest Sungnyemun Arsonist (KBS)
By Shay on February 11, 2008 at 7:52 pm
Update: South Korea arrests man in landmark fire (AP)
By Shay on February 12, 2008 at 5:19 pm
Update: Man ‘confesses to S Korea blaze’ (BBC)
By Shay on February 12, 2008 at 8:01 pm
Extra: Video of Namdaemun Collapsing (ZR5)