Thanks fer that, funnily enough, I was thinking abut these forts the other day. I first became aware of them as a nipper cos of the Arthur C Carke prog 'Mysterious World...In fact, here's the short vid.....
Vitrified Scottish Forts from "Arthur C. Clarke's Mysterious World" (1980) https://www.youtube.com/watch?v=jUb5212SCls