To the north of the dun territory, in Orkney and Shetland, Caithness and Skye, appeared a series of tall, tapering, stone towers, clearly designed as fortifications and known as brochs. As Euan MacKie has observed, these were the only advanced architectural buildings ever to be created entirely within prehistoric Britain, apart from Stonehenge. About 500 of them are known in northern Scotland where some of them once stood 9 m. (10 yd) or more in height and over 20 m. (22 yd) in diameter (plate 51). Each circular tower consists of an inner and outer layer of dry-stone walling tied together with a series of horizontal lintels which bridge the gap between them. In this space, which is about 1 m. (1 yd) wide, are a series of galleries superimposed one above the other and a slab-built staircase which climbs clock-wise to the top of the tower. The galleries were probably intended only to lighten the weight of the tower wall, thus allowing it to be built higher. The total thickness of the wall at its base was about 4.5 m. (5 yd).