Or, rather, a former prison. Seodaemun Prison History Hall is now a museum, principally featuring (1) the Japanese colonial government that constructed it in 1907, and (2) the Korean independence movement that fought them tooth-and-nail for 35 years.
Couple of things to note on the content of the museum.
Moderately graphic torture scenes were displayed with mannequins
It was basically just me and wave after wave of school classrooms
They kept using the prison right up until 1987, when they actually started doing a democracy (yeah, it was really pretty recent). Unrelatedly, the museum skips from 1945 directly to 1960.
I capped off the day with a city wall hike. A lot of the Fortress Wall of Seoul was destroyed during Japanese rule, but chunks are still around.
The end of the trail was some distance from the transit line to get home, so I sat down and tried to use the city’s bikeshare app again.