This a definite must do at Phnom Penh. This place is full of history and one of the few remaining prison that was left at its original state in the condition it was found in. It is pretty hard to imagine how people can still inflict such pain to their countrymen, and force out confessions from even young children. In the area, there are two of the 7 survivors seated near the middle of the compound and they would share the stories of the torture that took place. Of course, they would encourage you to purchase their autobiography.
Overall this place has a very dark gloomy presence casted over the high school and is truly an impressive place which will provoke your thinking. Tuk Tuk to here and the killing field out of the city, and finally back to your hotel would likely cost 15 USD for the entire day.