The Most famous prehistoric fort of the Aran islands is situated in Árainn at the edge of a cliff approximately a hundred meters high and dominates at the other side the area of Cill Mhuirbhigh. It's surely the most spectacular place of the island and the best moments to visit it is very early in the morning or After sunset when peace returns and the waves of the sea can be heard breaking up with strong violence to the base of the cliff. According to mythology, Aengus, son of the God Dagda and the Goddess Boann (personification of the Boyne river), was King of the Fir Bolg, a Celtic tribe who settled down on Aran and constructed these forts for protecting themselves from the inhabitants of the "continent". It's constituted by 4 series of concentric walls and the first ring is in some points 4 meters of height and are formed by 3 different levels. The original shape was presumably oval or D-shaped and it is thought that the ending part is collapsed for the continuous erosions of the sea, but do not exist written testimonies on the original shape of this fort. In the inside a large rectangular stone slab is visible that faces to the cliff: which was its function is not given to know. An interesting aspect of this fort is the massive stone Chevaux de Frise (this indicates a defensive system based on using slabs stone or wood that planted in the ground made the access difficult, especially to horses) that extends all around the third line of walls. In some points it's still very well preserved. There are still large doubts on the function of these forts, especially because from the inside it wasn't possible to have an easy view of the enemy that were approaching and therefore the hypotheses that considered these buildings to have defensive functions are uncertain; but the presence of the Chevaux de Frise makes clear that the access to the fort was wanted to be made difficult. Some studies suggest instead the use of this place for ceremonies, seasonal and magical rites made by the druids.