Before 1975, Toul Sleng was just an ordinary high school. The Khmer Rouge transformed the building in a prison and interrogation facility known as ‘S-21’. Prisoners were tortured over a period of many months before being sent to the Choeung Ka killing fields.

17,000 people were known to have been ‘processed’ through S-21 of which only seven survived.

The building is now a museum and memorial to those who suffered at the hands of the Khmer Rouge. A disturbing, but essential, part of any visit to Phnom Penh.