Holyhead Mountain has been continuously useful to humans since the middle Stone Age. The slopes show traces of field systems, dry-stone wall bases, and twenty surviving hut circles from the Iron Age settlement on the southwestern terrace - the Holyhead Mountain Hut Circles, occupied through the Roman period and possibly later. There are standing stones nearby, including the dramatic Penrhos Feilw pair south of the mountain. Caer y Twr, the late Roman watchtower on the east side, sits inside a prehistoric hillfort whose dry-stone ramparts are still visible. The Romans did not build the fort - they reused it, dropping their own watchtower inside walls that were already old when the legions arrived. Stand on the summit and you are standing on something that has been a marker, a refuge, and a lookout for fifty centuries.

On the northeastern side of Holyhead Mountain, the slope shows visible scars from one of the largest construction projects in Victorian Britain. Seven million tons of quartzite came out of those quarries between 1848 and 1873 to build Holyhead Breakwater - the longest breakwater in the United Kingdom, at over 1.7 miles, sheltering the New Harbour for the Dublin ferry trade. The stone was carried to the harbour on a dedicated railway and laid course by course by divers working in submarine bells. A brickworks attached to the operation produced specialist facing bricks, also from material taken from the mountain, and stayed in production until 1973. The breakwater would not exist without the mountain. The mountain still bears the marks of the breakwater. One physically rearranged the other.

The summit is rocky quartzite, and the slopes are clad in heather that turns the whole hill purple in late summer. Most visitors do not come for the summit. They come for what nests on the cliffs below it. The mountain ends to the southwest in the sea cliffs above South Stack, where puffins arrive in spring to dig burrows in the turf, where stonechats chip from the gorse, where Eurasian oystercatchers patrol the rocks with their loud orange-and-black severity. The RSPB runs a reserve at South Stack focused specifically on the seabird colonies. Grey seals haul out below in summer, visible from the clifftop paths if you know where to look. The combination - moderate hill, dramatic cliffs, large bird colonies, and an unbroken view across to Ireland - draws walkers and birdwatchers in numbers that the prehistoric inhabitants would have found incomprehensible.

There is a triangulation pillar at the summit, white concrete, weathered by Atlantic weather. From it you can see the entire western half of Anglesey, the breakwater curving out into the Irish Sea, the ferries running their tidy lines toward Dublin, and on the best days the smudge of the Wicklow Mountains sixty miles to the west. South Stack Lighthouse is visible to the southwest. Mynydd Bodafon, the highest point on the main island of Anglesey at 584 feet, looks smaller than it should from this angle. There is something disorienting about standing on the highest point in a county and being able to see clean across to a different country. Holyhead Mountain is the place where you understand, viscerally, why this island has spent five thousand years being the spot where the British Isles reach toward each other.