Venikelday http://www.themodernantiquarian.com/site/5087 is now seen "as a broch within a ring-work" [but Petrie saw it as resembling the Ring of Bookan http://www.themodernantiquarian.com/site/390 (which in the present state is comparing one unknown with another - locals remember it being a chambered mound!)] so obviously a ringwork is not solely Norman (the ringwork and bailey of England-shire) but a many-splendoured thing. And regarding Petrie's Bookan comparison archaeologist's are very hot to see this as an henge. So it might be best to see ringwork/ring-work as more of a description than a type of site.