Tuol Sleng, which means "Hill of the Poisonous Trees" was a high school in Phnom Penh before the Khmer Rouge took power in Cambodia in 1975. The Khmer Rouge quickly shut down all schools, and forced residents of Phnom Penh and other cities to the countryside in an effort to turn Cambodia into a pure agrarian state. Tuol Sleng, also known as S-21, was the regime's most important security prison. Some former classrooms were subdivided into groups of small individual detention cells, with passageways hacked out of the cement walls between them. March 1, 2012.