The Barracks is remarkable for two reasons. First, it's bloody cold. Second (and making up for the first), the Great Wall runs past and around it, forming its exterior wall. Here we can see one part of that exterior wall: to the left is the interior of the compound, to the right is the outside world.

The red areas are crabapples drying in the sun, a recurrent theme at Huangyagguan.

You can see this view from the other side in this photo.