If you have time to visit Siem Reap, you should visit Wat Thmey, which is famous for its killing fields. This is a memorial place and it has a different style in this area. There are English posters here, so you can learn about history and the Khmer Rouge regime. I never thought that there is a small killing field in Siem Reap. I visited this place as part of a city tour. No admission fee is required here. There is a monument from the Polport period with bones of people from the massacre. When I visited here during the Khmer New Year, commemorative activities were held here. I also witnessed people's charity and worship to the child monks. If you don't have a chance to see the real killing fields in Phnom Penh, this is a good place to substitute.
Truly depressing place, until you spy the local wildlife
A memorial dedicated to the war crimes of the Khmer rouge regime. There are several memorials all over Cambodia.