The Grianán of Aileach is a hillfort atop the 244 metres (801 ft) high Greenan Mountain at Inishowen in County Donegal, Ireland. The main structure is a 19th-century reconstruction of a stone ringfort, thought to have been built by the Northern Uí Néill, in the sixth or seventh century CE. 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.

https://en.m.wikipedia.org/wiki/Grianan_of_Aileach