S21 was a school converted into a prison. 1.5 million to 3 million people (over 1/4 of all Cambodians at the time) were killed between 1970-1979, either by Vietnamese guns, United States bombs.or at the hands of their own authoritarian governments.
Sightseeing in Hanoi, Tortoise Tower, Women’s Museum, Prison, and Water Puppets
All this can be seen in one day:Tortoise Tower, Women’s Museum, Prison, and Water Puppets. You get your tourism, history, and your culture in in one shot.