 It is at the head of a valley opening south to the sea, surrounded by a ditch over 8m wide and at present 1.8m deep.The fort's walls are up to 5.5 m (18 ft) high in places, 4 m (13 ft) thick at the bottom and 27.4 m (90 ft) in diameter.The inside is reached through a 1.8m passage roofed with double lintels.