A "kids stone fort"

This is along the lines of everyone building rock piles on Nagog Hill in Acton. But in this case, in an area that had two other large boulders with rock piles built on them, it seems as if the modern construction is made on top of an older one. Note the moss on the rocks in the 3rd picture.
What none of the pictures captures is that the rock outline is open at one end (just off the right edge of the last picture) and there is a short stretch of wall separate from the outline and dividing the opening in two parts. I took a video to show the whole structure as well as what appeared to be a crescent formed on the ground besides the fort, made of low single rocks buried in the leaves. But the video wouldn't download from the camera and I lost the details I would have liked to show. They suggest that this is not a normal kids fort. At a minimum, you'll admit, it is surprisingly well made.