Koh Ker was our next stop on the day trip to Preah Vihear, after we visited Beng Mealea. Our driver told us this layered pyramid was actually his favourite temple of Cambodia. The multi-day Angkor passes used to cover this site two, however it changed recently and we had to buy separate tickets. Not a problem, it was worth it. On the day of our visit, the site was mostly visited by local tourists - it's obviously a bit off the main fare. There's actually two-in-one ensembles: firstly, you enter through a set of ruins that looks something like three structures of Sambor Prei Kuk thrown together, similarily encircled by tree roots, a bit of a lake and general state of ruin-disrepair. You're already impressed, there are some nice photo ops. And then you walk some more and in the view in front of you a large pyramid appears.
It's square in layout, with layered stories one on top of each other and the best thing is the rickety wooden staircase you can take to the top and enjoy the view. It's couple of hundred steps, wear a hat and have some water, you will need it. But the view were absolutely amazing and the whole detour was definitely worth it! As Nan pointed out, the pyramid provides the most distinctive and unique feature of the whole ensemble.