Sierra de Guadarrama from the Roman road near El Escorial

The Roman road went slightly to the left of here and is now in private property but it is rejoined soon enough. To follow a line that is straight from an aerial photo isn't necessarily straight on the ground. The Romans were practical and didn't plough a path relentlessly through all obstacles. They would go round a hill if necessary!
My guess is that this wall was constructed partially from the paving slabs of the road.