A remote, very rarely visited but atmospheric site quite unlike any of the other temples in the Angkor area, Chau Say Vibol (also Chau Srei Vibol) must have been a large, imposing and important site in its day.
Now it's a a total ruin. The fact that it was clearly a strategic point, guarding the approaches to Angkor as well as the roads to Beng Mealea and Koh Ker, along with its fortress-like construction, seems to indicate destruction at the hand of humans rather than nature alone. Huge piles of massive sandstone blocks litter the ground on the top of the hill and ancillary buildings within the large complex lie in shattered mounds.
Some towers in the central complex do remain and the outer enclosure wall in laterite is still relatively intact, with four gopuras still evident. If you look carefully among the rubble you'll find some carvings with Hindu themes but basically this one is worth visiting for its unusual construction and atmosphere. Don't wander too far off the beaten track, be prepared for a lot of clambering over stones and watch out for snakes!
Chau Say Vibol is accessible by bicycle, motorbike and on a good day tuk-tuk or car. Take the road to Phnom Bok then hang a right — you may have to ask the locals for Prasart Chau Say Vibol. There's a rough track, which may or may not have been upgraded by now, which heads south passing through some scenic villages and eventually hitting the main Siem Reap-Phnom Penh highway near Rolous. The temple is situated on this track at the point where it hits the old Angkor to Beng Mealea road. So road conditions permitting you should be able to do a loop and return to Siem Reap from Rolous making for an interesting and off the beaten track Phnom Bok, Chau Say Vibol, Rolous circuit.)