I suspect that some of these are settlement first; defensive second, and some (I'm thinking of some of the more extreme promontory forts) are the other way around.
It's definitely worth someone having a think about the tree of other tags associated with "historic" and especially "historic=archaeological_site", because as it stands things are a bit confusing.
https://github.com/SomeoneElseOSM/SomeoneElse-style/blob/master/style.lua#L7126-L7290 currently covers how I'm handling it, but I bet filling in more details in some cases (such as the civilization for settlements) would be really helpful.