'The Broch Age' was roughly 500 B.C.E. to 500 C.E., but there were some around 700 B.C.E. and perhaps even as far back as 900. Of course quite a few could be built over even earlier structures, but archaeologists have a habit of only going as far back as the first monumental building and the ceasing excavating in order to preserve this ! This has been well argued by Dave Lynn in a paper yet to be published, along with a few examples of brochs known not to have been built on 'greenfield' sites e.g. The Howe

"'Caherdoon' which means 'stone fort stone fort'"

Bit like all those Knock Hills.

Some brochs are named "Dun", I think. Although I must admit that I don't know much about Brochs. But there's one on the Northern tip of Lewis called Dun Eistean, said to be the ancestral home of the Morrison clan.

Durham used to be called Dunholme which, if memory serves, is said to mean "the fort on the bend in the river". I wonder, though, what with the legends about Saint Cuthbert and the dun coloured cow. One thing seems very clear to me - and that's that the hill that now has Durham cathedral on it was of huge significance to the area's prehistoric population. I understand that there's some prehistoric stones to be seen in the Monk's Dormitory there, although I've never been because you have to pay to go in there and until recently I thought there was nothing but Christian stuff to be found there.