Allee Couvertes are Neolithic era gravesites, usually short little tunnels made of stone. This one, the Allee Couverte du Morgau Bihan is in Brittany, France. Kids were climbing all over it. I guess it's pretty hard to hurt stone after the third millennia or so.
That's the top of a home beyond the stones, which stand on private land, I believe. People stop there all the time. Those giant slabs of rock have been resting on the standing stones since around 2,500 B.C.
There are carvings inside the allee couverte too, but my pictures didn't capture them too well. A shepherd's crook, and what looks like spearheads or tools, possibly. A descriptive sign (in French, of course) called them palettes.
These structures predate druids and Celts by two thousand years, and nothing really is known about the people who set them up--except they liked to raise incredible stone monuments. Did they disappear, merge with the Celtic tribes that came later? In the case of the Allee Couverte du Morgau Bihan, I think the Celtic tribe would be Ossismi, since the site is near Quimper.