Stonehenge was behind a wall

In a recent documentary on Stonehenge I was surprised to learn that even though there were houses and facilities for large gatherings, most of the year nobody was home.

To me this sounded a bit odd, surely there would be guards to keep out squatters?

I now think there were, because it has been determined a large timber fence ran for 2 miles protecting the monument from one direction. Experts are saying it was to stop prying eyes from seeing rituals, others say it was to keep out drifting snow. While it may have served both of those purposes, I think it was more akin to the Berlin Wall - a year round barrier that could be manned by a handful of soldiers, regardless of whether there was a ritual happening or not.