It was once Tuol Svay Prey High School, before the complex's five buildings were cleared for their new role in 1975. The complex was renamed Security Prison 21 (S-21), barbed wire was added to the perimeter, and the windows were fortified with iron bars.
All the classrooms were transformed into prison cells and interrogation chambers. Each building has three floors at most. The bottom floor housed the smallest cells, each holding a single prisoner chained to the floor. Women were confined to the second floor. The third floor were mass prison cells, holding large groups chained to long iron bars.