From midnight to 1AM the travel channel had on a show about Stonehenge that I found entirely by accident, but filled me in on a lot of details about it.
It was constructed in 3100 BC, rebuilt into a different structure in 2150 BC, and rebuilt again into it's current form sometime between then and 1600 BC, when it was abandoned after a major change in religion in England that made it obsolete.
The Druids didn't exist until a thousand years later and have nothing at all to do with it. In fact, there have been several histories written about Stonehenge starting as far back as 1140 and continuing up to 1950, when they finally were able to properly date it with technology only then coming into existence.
On the last day of the 19th century (December 31, 1900) one of the largest stones fell over, and prompted the first actions to preserve what was left of it.
Like the Sphinx, it predates ANY form of written language by thousands of years, so a lot of it is still a guess, like the Skeleton they found under one of the stones that had been beheaded by a sharp metal blade, a thousand years before such a thing was supposed to exist.