Brochs are the most spectacular of a complex class of round house buildings, and there is now little doubt that these towers are unique to Scotland. They vary from 16 to 50 ft (5 to 15 m) in internal diameter, with around 10 ft (3 m) thick walls. The only surviving complete broch in Scotland, at Mousa in the Shetlands, stands 42 ft (13 m) tall.

Broch walls are almost always double skinned, but were tied together with linking stone slabs, which probably could have served as steps to higher floors (known as galleries). These slabs may also have been access ladders to the roof. Most brochs have ledges around the inner walls known as scarcements which would have supported timber floors. Some brochs may have had second and even third floors.

Just inside the main entrance there was usually a guard chamber, and sometimes even two. Large stones for sealing off entrance passageways have been found near guard chambers. It is now generally accepted that brochs were roofed, probably with a conical timber thatched frame. Quite a number of brochs, particularly those in Orkney, have outbuildings surrounding the main structure.

Many brochs were built as extensions onto previous stone round houses, and it is likely that their uses would have changed through the centuries, with further extensions and outbuildings being added later. Such prestigious buildings would no doubt have become much sought after as homes for prominent Scots, and perhaps were instrumental in the founding of the Clan systems.