What an awesome little MOC; this one called 'Ruins' is by Patrick Massey, and it utilizes some great techniques that we will discuss today.
As well as the awesome rock work made up of slopes positioned sideways that look very naturally incorporated into the landscape, the perfectly circular towers are a huge highlight. From what I can see (although it's hard to tell), he has used a combination of bars, 1x1 studs and modified plates that allow the tiles to be angled outwards.
Make sure to follow Patrick on Flickr here from very similar awesome MOCs: https://www.flickr.com/photos/94545380@N06/14129227047/in/photostream/