This is a sign right as you enter S-21. S-21, or Security Prison 21, is a former high school, which was used by the communist Khmer Rouge regime. The Khmer Rouge renamed the complex "Security Prison 21" (S-21) and construction began to adapt the prison to the inmates: the buildings were enclosed in electrified barbed wire, the classrooms converted into tiny prison and torture chambers and all the windows were covered with iron bars and barbed wire to prevent prisoner escapes.