Perched on a cliff overlooking the Atlantic Ocean, Dun Aonghasa is the largest of the prehistoric stone forts of the Aran Islands. It is enclosed by three massive dry-stone walls and a chevaux-de-frise consisting of blocks of limestone set vertically into the ground to deter attackers. Originally constructed c.1100BC, it was re-fortified around 700-800 AD. Excavations revealed significant evidence of prehistoric metalworking, as well as several burials. Visitor safety information: The fort is about 1km from the Visitors Centre and is approached over rising ground. The last section of the path is over rough, natural rock and care is needed, especially when descending. Boots or strong walking shoes are recommended. There is no fence or barrier at the edge of the cliff.