The retaining wall aspect makes it more likely to be a natural formation, as we see only one side of the “wall”, the other side hidden by earth.
So we don’t know how “thick” the “wall” is, or how large the stones actually are, because we don’t know how far they extend on the other side.
Definitely some excavation is in order. Expose the top blocks to see how far they go into the mountain. If they are manmade, they are going to be relatively rectangle, maybe 50’ wide. However, if they are natural, they could be hundreds of feet wide on top.