How'd they do that

Because I was studying abroad there was some short excursions that were offered to the Americans. I had to get up so early in the morning to catch the charter bus to get on our way. The weather was so cold and windy. When we arrived at Stonehenge it wasn't crowded at all. We went in the morning right as it was opening and no one was there. Apparently, you used to be able to climb around the stones, but not anymore. there is this thin rope that is led all around. It was so breath-taking walking around the structure. To think that it was built 1000s of years ago just baffles me. How did they do that? They used Stonehenge as a clock, and also a Calendar to tell you what month it was. I can't wait to go back and visit!!!!