High in the Cambrian mountains of mid Wales, perched on a slope above the chaotically youthful river Afon Tarenig, the bleak aspect of the Roman fort at Cae Gaer speaks of military expediency and urgent purpose.

In the sunshine of early spring it looks almost serene. But to a newly arrived legionary, in the depths of winter, immersed in an alien landscape still home to wolves and bears, it must have felt like the edge of the world.

Surrounded by steep hills, the location of the fort is an elaborate compromise. Close to fresh water and commanding the junction of two valleys, the site would have forced an enemy to take a long and uncomfortable diversion around it. Every detail of the foe’s military life, however, would be visible to a watcher concealed on the slope above.

A roughly square enclosure, the fort was built from turf ramparts topped by a wooden palisade. Nearly 20 centuries later the banks, densely covered now in heather and coarse tussock grasses, are still surprisingly intact, and the ditches outside them continue to be a significant obstacle to the aspiring visitor. Which, as my companion pointed out, was the whole idea.

Enclosing about a hectare of poor, waterlogged land, the fort is a small, basic structure, perhaps associated with the campaign of Quintus Veranius Nepos in the winter of AD 57. This newly appointed governor of Britain set out to subdue the intransigent Silures tribe, early masters of guerilla tactics, but didn’t survive to complete his task.

I stood close to where the northern gate had been, wondering what it would have been like to stand guard here long ago, perhaps relieved by the news that a mission had been stalled by the death of the governor and that soon we would be pulling back to the new garrison at Viroconium.

Bordered now by bland commercial forestry, the fort endures as a pale pattern in the landscape briefly visible from the fast, twisting trunk road to the coast. Passed by, in every sense, it lingers as a footnote on a footnote of history.