A typical broch is 20 metres in diameter, with 3 metre thick walls. On average, the walls only survive to 4 metres. The distribution of brochs is centred on north west Scotland although isolated examples occur in the borders and near Stirling.
More often than not, the walls are hollow, containing flat storage spaces (called galleries or cells) and steps to higher floors. Beside the door, it is normal for there to be a cell breaking off from the passage into the central space; this is known as the guard cell although there is no evidence it had any defensive value.
On the Orkney and Shetland Islands, there are very few cells at ground floor level. However, brochs in this region have scarcement ledges which would have allowed the construction of a very sturdy first floor.
Brochs were always placed in locations which were easily defended, close to arable land and a source of water (many have deep wells or natural springs rising within their central space). They are often built beside the sea and on the site of previous dwellings such as roundhouses.
Early in the use of a broch (from the middle of the 1st millennium BC until the early 3rd century AD) they would be used purely as defensive structures, places of refuge for communities and their livestock. As the Iron Age slowly gave way to the early Medieval period, however, it seems the defensive value of the broch design was lost. They became the Stately homes of their time, objects of prestige and superiority for rich merchants.
Some good examples of brochs exist at Mousa on Shetland (the walls here are fully intact), in Glenelg (a galleried dun can also be seen here) and at Dun Carloway on Harris. The best brochs in the Orkney Islands are at Gurness and Midhowe.