Norman’s Law (the hill of the northern men) is in height 850 feet above the sea level. It commands a most delightful prospect, especially to the north, where the Carse of Gowrie and the Frith of Tay appear in full view in all their richness and variety. There are three concentric circles of rough stone near the top, supposed to have been a fortification of the Danes to cover their inroads into the country, or perhaps erected by the natives to repel these invaders.
From The New Statistical Account of Scotland By Society for the Benefit of the Sons and Daughters of the Clergy (1845) v11, p49.
It seems from the RCAHMS record that the two outermost rings are the oldest, and delineate the prehistoric hillfort. The innermost possibly postdates the Roman invasion and was built to protect the huts of the settlement within.