The Grianan of Aileach, sometimes anglicised as Greenan Ely or Greenan Fort, is a hillfort atop the 244 metres (801 ft) high Greenan Mountain at Inishowen in County Donegal, Ireland. The main structure is a stone ringfort, thought to have been built by the Northern Uí Néill, in the sixth or seventh century CE; although there is evidence that the site had been in use before the fort was built. It has been identified as the seat of the Kingdom of Ailech and one of the royal sites of Gaelic Ireland. The wall is about 4.5 metres (15 ft) thick and 5 metres (16 ft) high. Inside it has three terraces, which are linked by steps, and two long passages within it. Originally, there would have been buildings inside the ringfort. Just outside it are the remains of a well and a tumulus.
Grianán of Aileach
An Grianan fort at sunset
Sunset over the fort
Plan of the Grianán
A hillfort is a type of fortified refuge or defended settlement located to exploit a rise in elevation for defensive advantage. They are typical of the late European Bronze Age and Iron Age. Some were used in the post-Roman period. The fortification usually follows the contours of a hill and consists of one or more lines of earthworks or stone ramparts, with stockades or defensive walls, and external ditches. If enemies were approaching, the civilians would spot them from a distance.
Maiden Castle in England is one of the largest hillforts in Europe. Photograph taken in 1935 by Major George Allen (1891–1940).
The ramparts of the multivallate British Camp in Herefordshire
Exterior view of the Ringfort Grianan of Aileach situated in County Donegal
Hillfort at Coaña, Asturias, Spain