The Iron Age hillfort of Caer y Twr crowns the summit of Holyhead Mountain. A drystone rampart, in places still two metres high, encloses an area of about seventeen acres on the eastern side of the peak. The structure dates from the first millennium BC, and the Romans later added a watchtower of their own to keep an eye on the Irish Sea raiders who terrorised the coast in the late Empire.
The fort has not been excavated in any depth, which is part of its appeal — you walk over a defensive system that has been quietly weathering for two and a half thousand years, with nothing in the way of signposts or interpretation. The walls follow the natural contours of the ridge so closely that it takes a moment to register them as built rather than tumbled rock.