I’ll check the library. But the piles of rock that make up the “walls” there wouldn’t hold livestock in, much less attackers’ out. They’re not even much good as cover. Maybe they were better built and higher once?
Keeley doesn’t talk about that site in particular but about the tendency among archaeologists to assume that primative people are peaceful and that things serve a non-warlike purpose by default in general. It’s possible that this particular site isn’t a fortification but don’t assume that what you see now is all that was ever there. A lot of primative fortifications were built of mixed materials that included gravel, dirt, clay, wood, and other materials that can decay or wash away. Stone is heavy and hard to work and thus requires a lot of effort to build with. England was once covered with small castles and keeps made of wood, none of which have survived. All you’ll see now are mounds of dirt that were once part of the fortifications.