Built from drystone walling, brochs vary from five to fifteen metres internal diameter, with three metre thick walls. 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 in some cases serve 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 brochs in the isles that guard cells in entrance passageways are close to large door-check stones.

Brochs are roofed with a conical timber framed roof covered with a locally sourced thatch. The source of this structural timber isoften bog and driftwood where other more plentiful sources are unavailable. Most brochs have scarcements (ledges) which allow the construction of a very sturdy wooden first floor and some even have stairs on the first floor, which head upwards to a second story. Some brochs such as Dun Dornaigill and Culswick on Islay have unusual triangular lintels above the entrance door.

Brochs are often located close to arable land and a source of water (some have wells or natural springs rising within their central space). Sometimes, on the other hand, they are sited in wilderness areas. Brochs are often built beside the sea; sometimes they are on islands in lochs. Those built beside the sea can be seen on imposing cliff faces.


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