Who built this?

The reason I ask is that I've never figured out this wall at the back of the car park of our modern flats. It's obviously part of a wall of a larger building that is at least pre-Victorian. The wall backs directly on to a steep earth bank, so there could never have been anything on the other side of it. So, why the lintels? (i.e. the long horizontal stones.) Recently I have learned that there was a mill on this site, so I can only presume that this is what's left of it.