The name Treago is half Welsh: "Tre" means homestead or farm, "Ago" comes from Iago, the Welsh form of James. There had been a Welsh-named dwelling on this ground before the present house went up. Around 1500 the Mynors family demolished it and built afresh, in a tight quadrangle of locally quarried stone, with corner towers at all four angles and the rooms turned inward to face a sheltered courtyard. The outer walls were almost blank: no windows facing the open ground beyond, no easy targets for archers or hand-gunners on the Welsh side of the border. The closest comparison is Croft Castle a few miles north, also in Herefordshire. In 1975 the chief inspector of ancient monuments, A. J. Taylor, noticed something striking during a visit: mason's marks on Treago's stonework matched those at Raglan Castle in Monmouthshire, and could be traced back to William Herbert, 1st Earl of Pembroke. The same craftsmen, working a few decades apart, may have shaped both places.

The Mynors are not aristocracy in the dukes-and-marquesses sense. They are something more interesting: a country gentry family who simply refused to let go of their house. Some Mynors records reach back to the eleventh century, though the direct association with St Weonards is documented only from the mid-fifteenth, just before the present castle went up. The family eventually acquired a baronetcy: the Mynors of Treago Baronetcy is currently held by Sir Richard Mynors, born 1947, who lives in the house with his wife Lady Fiona Mynors and their three daughters Alexandra, Frances, and Victoria. Sir Roger Mynors, the distinguished Oxford classicist, spent his last years here. Sir Richard is a working vintner; Lady Fiona an educational consultant. They moved into the main house in 1989 and have spent the years since modernising it without losing it, undoing two earlier neglected centuries of half-finished improvements.

Around the castle, the eighteenth century landscape was a typical English parkland, neat and pastoral, then largely forgotten between 1790 and 1840 as fortunes shifted and family attention drifted. From the 1840s onward, four big projects rebuilt it. A walled garden was set near the road, separated from the house by a shrubbery whimsically named The Wilderness. A walled kitchen garden was laid out south of the house, where the vineyard now grows. A Tudor-Italianate formal garden was added with clipped Irish yews, gravel paths, and flower beds, marked on tithe maps as Treago Garden. And a new principal driveway swept in from the east, replacing the old approach. Since 1991 the roses planted across the grounds have been mostly David Austin English roses, the modern hybrids bred to look antique. A small fountain put in in 1990 carries the Mynors family crest, a man's hand holding a bear's paw, the heraldic visual pun on the French main ours, which sounds like "Mynors."

In 1932 the Great Western Railway named one of its Castle Class locomotives, No. 5019, Treago Castle. The engine ran the network for thirty years before being withdrawn in 1962. The naming was hardly coincidence: Treago was a working country estate then as it is now. The original stables, behind the main house, were converted in the 1990s into three connected holiday cottages, Hollyhock, Coach House, and Looseboxes, that still take week-long bookings. The old quarry behind the buildings holds the vineyard that yields Treago's award-winning red, white, and sparkling wines. A small Japanese garden survives in fragments. An indoor heated swimming pool was added during the 1989 renovations. Heating now runs partly off a ground-source geothermal pump and partly off a biomass boiler. The fortified manor that was built to keep the Welsh out has, five centuries on, opened a north porch to make the house an all-year-round home.