When going to visit the Killing Fields, the best sequence is S-21 Prison first, then the Choeung Ek actual Killing Fields. This way you get the stories of the prisoners first, then follow the path to how they died. I did this the other way around which worked logistically but back tracked any emotional impact. What got me the most was that this prison was a high school and the classrooms became the torture chambers. The regime photographed each prisoner and these pictures are on display. You are not allowed to take pictures of the photos but can be seen at the killing fields website.
The rules outside the interrogation rooms.
About this building.