I mean, sure. You could probably turn a lot of things into ramshackle foritifactions if you tried hard enough. You can circle the wagons, board up doors, whatever. But I don't think this was what Stonehenge was intended to do.

First up: the bluestones. Other than the large stones (called sarsens) there were several stones of foreign origin called bluestones, which were comparatively smaller at only a meager 2 to 4 tons. Why would you drag the bluestones to your hypothetical fort? All they would do would be to take up space better used to store food or people.

In addition, Stonehenge was constructed around 2000 or 3000 BC, with the earliest earthworks surrounding it (a moat and ditch, which could be possible support for this theory, but I doubt it) and this would have been late Stone Ages, early Bronze Ages for the people of Britain. That's not to say the idea of fortifications did not exist by this point, for example just a bit north in Scotland a large stone wall existed around the Ness of Brodgar, around 3200 BC. So defensive walls were not a foreign concept to the people of primeval Britain, assumably. If so, why didn't they just build normal walls? What in god's name would possess them to lift massive, unwieldy stones with painstaking labor to make a wall with holes in it? If they were caught by surprise, the stones wouldn't do anything because there were gaps between them! Sure, you could roll up logs (wouldn't you have to work the logs to make them fit in the gaps, though?), but there's another question: Why these stones? Why use move massive stones miles (including the bluestones which would have no real purpose in a fort) when you could presumably use more local rock to get the job done if all you wanted was a mundane defensive structure?

Finally, purpose. We admittedly do not know much about the purpose of Stonehenge. Human remains have been brought up around Stonehenge, some cremated. Sure, this could be a sign of battle, but I'd point to a religious function if I had to guess. Not to mention our so called fort is aligned to the summer and winter solstices. Why align a fortress to the solstices? Seems pointless. The area may have been believed to have healing properties, or used for ancestor worship. Nearby is the Archer of Amesbury, the remains of a man who owned several items of gold and copper along with many arrowheads (hence the name). He is also possibly foreign, believed to be from an Alpine region. Sure, he could be some invader from continental Europe, but if so, why bury him with his goods? Second, he was injured, with a bad knee. Some scholars believe that he many have come to Stonehenge for the above mentioned healing properties, and was buried nearby.