This amphitheatre-like stone fort encircles the top of Grianán Hill like a halo with eye-popping views of surrounding loughs. On clear days you can see as far as Derry. The fort may have existed at least 2000 years ago, but it's thought that the site itself goes back to pre-Celtic times as a temple to the god Dagda. Between the 5th and 12th centuries it was the seat of the O'Neills, before being demolished by Murtogh O'Brien, king of Munster.
Most of what you see now is a reconstruction built between 1874 and 1878.
At the foot of the hill is merry-go-round-shaped Burt Church. Built in 1967, it was modelled on the fort by Derry architect Liam McCormack.
The fort is 18km south of Buncrana. Its small arena can resemble a circus whenever a tour bus empties out inside the heavily restored 4m-thick walls.