Page "Broch" Paragraph 15

Brochs vary from 5 to 15 metres ( 16 – 50 ft ) in internal diameter, with 3 metre ( 10 ft ) thick walls.

On average, the walls only survive to a few metres.

There are five extant examples of towers with significantly higher walls: Dun Carloway on Lewis, Dun Telve and Dun Troddan in Glenelg, Mousa in Shetland and Dun Dornaigil in Sutherland, all of whose walls exceed 6. 5 m ( 21 ft ) in height.

Mousa's walls are the best preserved and are still 13 m tall ; it is not clear how many brochs originally stood this high.

A frequent characteristic is that the walls are galleried ( the outer and inner wall skins are separate but tied together with linking stone slabs, with an open space between ).

These linking slabs may in some cases have served as steps to higher floors.

Beside the door, it is normal for there to be a cell breaking off from the passage ; this is known as the guard cell.

It has been found in some Shetland brochs that guard cells in entrance passageways are close to large door-check stones.

Though there was much argument in the past, it is now generally accepted that brochs were roofed, probably with a conical timber framed roof covered with a locally sourced thatch.

The evidence for this assertion is still fairly scanty, though excavations at Dun Bharabhat, Lewis, have supported it.

The main difficulty with this interpretation continues to be the potential source of structural timber, though bog and driftwood may have been plentiful sources.