Perched on a cliff overlooking the Atlantic ocean, Dún Aonghasa is the largest of the prehistoric stone forts of the Aran Islands. The fort consists of three massive dry-stone walls and a chevaux-de-frise, a dense band of jagged, upright stones, surrounding the fort from cliff to cliff, designed to impede attackers. As this site is managed by the OPW, school tour visits are permitted free of charge provided certain conditions are met.