Sitting atop an 800 foot high mount, Grianan is a ring fort built in the sixth or seventh century. It served as a central fort for the Aileach royalty during the Gaelic social structure of the country, in which kings ruled in an order of hierarchy throughout the lands. The wall of the fortress measures 16 feet tall and 15 feet thick, with three terraces within the walls. Residential buildings are thought to have existed inside the fort early on. Grave sites and a well we’re discovered at the site. It has been proposed that Grianan of Aileach may be the oldest building in all of Ireland.