Barmkin Hillfort at Echt, Aberdeenshire. Large hillfort with five concentric defenses. The innermost wall encloses a circular space of about 122 m in diameter. Hillforts of this type are power centers in a hierarchical and tribal society; they appear in the Late Bronze and Early Iron Ages (800 - 100 BC). The five concentric defenses certainly represent several phases of fortification and possibly reflect changes in siege techniques such as the introduction of the sling. Scotland.