Formerly the Chao Ponhea Yat High School,[2] named after a royal ancestor of King Norodom Sihanouk, the five buildings of the complex were converted in August 1975, four months after the Khmer Rouge won the Cambodian Civil War,[3] into a prison and interrogation center. 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 windows were covered with iron bars and barbed wire to prevent escapes.

From 1975 to 1979, an estimated 17,000 people were imprisoned at Tuol Sleng (some estimates suggest a number as high as 20,000, although the real number is unknown). At any one time, the prison held between 1,000–1,500 prisoners. They were repeatedly tortured and coerced into naming family members and close associates, who were in turn arrested, tortured and killed. In the early months of S-21’s existence, most of the victims were from the previous Lon Nol regime and included soldiers, government officials, as well as academics, doctors, teachers, students, factory workers, monks, engineers, etc. Later, the party leadership’s paranoia turned on its own ranks and purges throughout the country saw thousands of party activists and their families brought to Tuol Sleng and murdered.[2] Those arrested included some of the highest ranking communist politicians such as Khoy Thoun, Vorn Vet and Hu Nim. Although the official reason for their arrest was “espionage”, these men may have been viewed by Khmer Rouge leader Pol Pot as potential leaders of a coup against him. Prisoners’ families were often brought en masse to be interrogated and later executed at the Choeung Ek extermination center.

In 1979, the prison was uncovered by the invading Vietnamese army. In 1980, the prison was reopened by the government of the People’s Republic of Kampuchea as a historical museum memorializing the actions of the Khmer Rouge regime.

The establishment of the Royal Palace in Phnom Penh in 1866 is a comparatively recent event in the history of the Khmer and Cambodia. The seat of Khmer power in the region rested in or near Angkor north of the Great Tonle Sap Lake from 802 AD until the early 15th century. After the Khmer court moved from Angkor in the 15th century after destroyed by Siam, it first settled in Phnom Penh which back then named as Krong Chatomok Serei Mongkol  in 1434 (or 1446) and stayed for some decades, but by 1494 had moved on to Basan, and later Longvek and then Oudong.The capital did not return to Phnom Penh until the 19th century and there is no record or remnants of any Royal Palace in Phnom Penh prior to the 19th century. In 1813, King Ang Chan (1796–1834) constructed Banteay Kev (the ‘Crystal Citadel’) on the site of the current Royal Palace and stayed there very briefly before moving to Oudong. Banteay Kev was burned in 1834 when the retreating Siamese army razed Phnom Penh. It was not until after the implementation of the French Protectorate in Cambodia in 1863 that the capital was moved from Oudong to Phnom Penh, and the current Royal Palace was founded and constructed.