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.