Morning, visit to the Royal Palace, built by King Norodom in 1866 on the site of the old town. Visit the Silver Pagoda, located within the grounds of the Royal Palace, the Silver Pagoda is so named because of its floor, which is made up of 5000 silver tiles.
You then continue your visit at The notorious Choeung Ek Killing Field and Tuol Sleng Genocide Museum afterward.

• Choeung Ek Killing Fields, 15km southwest of Phnom Penh, is the site of brutal executions of more than 17,000 individuals, most of whom first suffered through interrogations, torture and deprivation in Toul Sleng Prison during Pol Pot regime (From 1975–1979).
• Tuol Sleng Genocide Museum (S21 prison). This prison was a high school, and used as a prison by Pol Pot’s security forces and became the largest center for detention and torture during the rule of the Khmer Rouge.

You can visit Wat Phnom, the most famous buddhist temple in Phnom Penh, instead of Choeung Ek Killing Field and Tuol Sleng Genocide Museum, if you feel they are not suitable to you. Wat Phnom was built in 1373, and stands 27 metres above the ground. It is the tallest religious structure in the city. The pagoda was given the name of Wat Preah Chedey Borapaut.

Afternoon, get a transfer to Siem Reap. The transfer takes approximately 6 hours.
Stay overnight in Siem Reap.