Fascinating if over-detailed book (Watkins' 'The Ley Hunter's Manual' is I think better). Poor Watkins is misrepresented by both friends and enemies - the latter mostly professional archaeologists.
Trying to summarise: Watkins realised (he was among other things a commercial traveller around Herefordshire, and was well aware of the problems of finding one's way round, and the importance of landmarks) that prehistoric man had a problem of transport. To take one example: salt. There are local deposits of this in Britain; but projecting time backwards, how could the stuff be moved around? There were no motorways, or even roads; no tarmac; no motor traffic; no bikes; no maps; not even weedkiller to keep paths clear... obvious points which many people seem unable to grasp.
Watkins' theory was simply that straight tracks were laid out by line-of-sight and marked by whatever method was feasible - dug-out notches on the skyline (early man could do earthmoving on quite a scale), upright stones arranged in pairs to point the way, perhaps church steeples, large stones by the pathside - of types not found locally, to remove doubt. Watkins thought some large flat stones marked with cup and ball marks might be in effect maps of local 'hill forts'. He thought Silbury Hill was built specifically as a landmark. Trees were another possibility, though obviously they would be visible now, if at all, only by traces.
Another of Watkins's examples was water: springs of clean water were presumably a useful asset (and some contained health assisting minerals, though obviously we're in eras predating chemical knowledge). Paths to them might be marked out.
And much more in this vein, including signalling by means of beacon fires.
His SUPPORTERS have often taken a description by Watkins of a sudden insight into this possibility ('wires.. across the countryside') in an electrical sense, adding a whole assemblage of material on sacred sites, lights, currents, electric charges and shocks, and what have you. And of course there was a temptation to rule lines on the then-new Ordnance Survey maps. They also renamed as 'ley lines' what Watkins christened 'leys'.
His OPPONENTS generally laid into the detail - place-names for example obviously are a high-risk source of evidence. So are buildings - many 19th century churches are built in mock-old styles, many manor houses aren't reliable indicators of archaeological precedents, etc.
I think there was also a class element here: archaeologists like, or liked, to look at palaces, military structures, cathedrals, massive megaliths, impressive graves, treasure hoards, and generally high status things. Watkins tried to redirect attention to humble practical tracks and paths.