The people who built Caer y Twr made a choice with a sweep of view few defensive positions in Britain can match. The summit of Holyhead Mountain is the western lookout of north Wales -- a granite knob rising from a coastal plain, with cliffs dropping toward South Stack on one side and a long slope back down to the modern town of Holyhead on the other. They built a defensive enclosure among the rocky outcrops at the top, taking advantage of the natural geography. Some archaeologists have suggested the site served less as a defended settlement and more as a watchtower, a signal post, or possibly even a lighthouse beacon -- some kind of high warning station rather than a place people actually lived. The interior is small, the position is exposed, and very few hut foundations have been found at the summit itself. The settlement, if there was one, lay further down.

Most of Caer y Twr is now rubble. The walls have collapsed and weathered for two thousand years. But the line of the defences is still legible. On the north and east sides, a large stone rampart survives -- reaching three metres at its highest point, faced with carefully placed stones, the kind of work that takes a community a generation to complete. The entrance was through a rocky gully, the natural cleft in the summit, where the original gate would have been controlled by a wooden barrier and watchful sentries. Excavation at the site uncovered the footings of a tower from the second to fourth centuries -- well after the original Iron Age construction, in the Roman period, when the higher of Anglesey's lookout positions was put back into military use. The tower was probably a signal station for the small Roman fort at Caer Gybi down at the harbour. A flame on the summit on a clear night would have been visible for tens of miles in every direction.

Below the summit, on the slope at the eastern foot of the mountain, lies a group of hut circles called Cytiau Ty Mawr -- the Holyhead Mountain Hut Circles, or sometimes the Irishmen's Huts in local English. These are mostly third and fourth century -- contemporary with the late Roman use of the watchtower above -- and contain enough preservation that the floor levels, the hearths, and the stone shelves of individual houses are still visible. Twenty round huts have been identified, with several rectangular structures alongside. The pattern is one of a small farming community, probably with mixed Romano-British and native British inhabitants, working the land in the shadow of the fort. Some of the huts still hold the small details of household life -- a fireplace in the centre, a built-in shelf in the wall -- the kind of intimate archaeology that connects the visitor directly to the people who lived there.

Caer y Twr is reached by a footpath that climbs from the RSPB visitor centre at South Stack, on the western edge of Holyhead Mountain, or from the lower car park to the south. The walk is steep in places but not technical. The summit takes about an hour to reach from the car park if you do not stop. Most people stop. On a clear day the view is reason enough -- the lighthouse at South Stack flashing below you on its little island, the cliffs running away to the north, the Wicklow Mountains a faint blue line across the channel. The hillfort is open access and free to walk. There is no interpretation board at the summit itself; you have to know what you are looking at to recognize the rampart. Below, by contrast, the hut circles have been excavated and signed and are part of a heritage trail. The whole site is a scheduled monument.