On one of the many trips to Seoul Sara and I have taken we chose a cold, overcast day with light drizzle to visit Seodaemun Prison. I would list this as one of the top places to see if you are living in or visiting Seoul. There is a lot of information in English, and doing a little background reading on the independence movement and some key figures before visiting would be beneficial to you.
The prison was used to house Korean prisoners during Japanese colonial rule between 1910-1945. The Japanese tortured and executed prisoners here. The memorial outside lists the names of 90 know Koreans who lost their lives in the prison, although total deaths is estimated to be more than 300.
The museum is riddled with information, although sadly a lot of it is not in English. Regardless of that fact I learned a lot more about the hardships and conditions that prisoners had to endure during this dark point in Korean history.