This and whipangel's post seem to be converging on the same point (sorry :-) You've probably already seen this -
"Atkinson's medieval evidence was no less striking. Small postholes containing iron nails, early medieval potsherds and a silver coin of Ethelred II dating to 1010 suggested that the terraces had been revetted by posts. He also found an iron spearhead, and explained the evidence as defence against the Danish invasions - not unreasonably, given the skirmish at nearby East Kennet in 1006 reported in the Anglo-Saxon Chronicle. He concluded that the original terraces had been recut and fortified. The postholes, however, are located on the inside of each terrace and therefore imply revetment rather than defence."*
Thing about the Anglo-Saxon fortifications idea is that there doesn't seem to be enough archaeological evidence to support it. Also, although at first sight Silbury looks like a great place to build a fort... is it? There's no water up there, not much space to support livestock or the defenders and their families. The attackers on the other hand have all those things and could just lay siege to the place. Could see Silbury being used as a lookout though - with perhaps a beacon up there as well?
