Seodaemun Prison is a sad and brutal reminder of the 35 year long colonial rule of Korea by Japan. This issue still resonates today becase Japan and Korea often cannot carry out basic diplomacy between each other.
Japan has apologized on multiple occassions, but many Koreans feel the sincerity is lacking. While Japanese right-ring politicians (and sometimes Prime Ministers) flame the fires in Korea by claiming the "comfort women" issue is not true, or by visiting the Yasukuni Shrine which honors some Imperial era war criminals.
I will not delve into the current politics, but Seodamun Prison does reflect a thorn in relations that even today is not easily resolved. Most Japanese buildings in Korea were torched or demolished the days following Japan's defeat in World War II. Indeed, nearly 1,000 Shinto shrines in Korea became bonfires as celebrations swept the country. Korean's feeling resentment for the years of bowing to shinto shrines, being banned from speaking Korean, and the forced adoption of Japanese names were finally free to unleash their rage. In 1996, with much fanfare one of the last symbols of Japan's colonial rule (the Government General Building) was destroyed on live television. Japan had offered to pay for the buildings restoration or removal cost and transportation to Japan, Korea refused and the demolition began. Seodaemun survived in large part, because its use as a prison continued all the way until 1987! There are few places left in Korea where you can see and explore Japanese colonial buildings, the Gunsan port area perhaps being the best preserved district.
Seodamun Prison Hall is worth visiting, but I must warn there are gruesome diplays of torture. In Korea some school groups visit the site, but I must disagree wholeheartedly that young children be allowed here. Children below middle school cannot understand how or why this happened. For those who think the displays do not reflect the real history, please feel free to research.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.