The _Grianan of Aileach_, the ancient seat of the O’Neills, kings of Ulster, is a fort on the summit of a hill 803 feet high, about 5½ miles from Derry, overlooking the east shore of Lough Swilly. It consists of three extensive circular ramparts, greatly dilapidated, formed of earth mixed with stones, the outer enclosing an area of 5½ acres. Within the inner breastwork is a cashel, a circular wall about 17½ feet high, enclosing an area of 77 feet in diameter. It has an average thickness at the base of 13 feet, and is not quite perpendicular, but has a curved terrace-shaped slope, like Staigue Fort in Kerry. On each side of the entrance gateway are passages within the thickness of the wall, extending in length to one-half of its entire circuit. These do not communicate with the gateway, but have entrances from the area at their northern and southern extremities. The passages are 5 feet high, and over 2 feet wide, with sloping sides, the roof being formed of stones laid horizontally. The entrance gateway is 4 feet 3 inches wide at the base; the sides are inclined, and the original height was probably 6 feet. On each side is a reveal intended to receive the doors which barred the passage. This great residence was destroyed by Murkertagh O’Brien, king of Munster, in 1101. The Cashel was restored by Dr. Bernard, of Derry, during the years 1874–8. Out of the mass of myths and legends concerning primitive times in Ireland, it is invariably difficult, if not impossible, to disentangle the slight threads of fact from the web of romantic fables of bards and chroniclers. These have their value, however, when supported or tested by the evidence gleaned in the actual field of archæological investigation. Isolated geographically as Ireland was, she was yet the shore upon which the successive waves of influence sweeping over Europe spent themselves. She was, as a rule, not only late in being brought under the sphere of new influences, but some had especial opportunity of development, and many retained their hold and flourished long after they had ceased to exist elsewhere. The extent of the forts of Ireland, and the length of time they remained in occupation, is an example of this.[89]