Plas Allt-lwyd was the home of the Hughes family of Rhos Tyddn, Devil's Bridge. Built shortly prior to 1831 by Haycock of Shrewsbury, by the 1980s the plas became a hotel, renamed Plas Gwyn. A news item in the Western Mail published ahead of its sale in 1985 described it as comprising nine letting bedrooms and two ground-floor bedrooms, two bathrooms, conservatory, child's playroom, main bar, lounge, living room, office, dining room, kitchen, scullery, cellar wine bar, beer cellar and barrel store, and beer garden. The building is now (2018) a residential care home.

The building has well-preserved Regency-style features, and notable internal fittings include three plain neo-classical pilastered marble fireplaces in the bar, lounge, and dining room, each with five different types of coloured marble, as well as wrought-iron gates leading from the main bar to the lounge and an arched entry to the hall with half-glazed double doors and a fanlight with radiating bars and coloured glass margins. The staircase is broad and built of oak with a mahogany moulded rail and slim twisted balusters.

Externally, the house comprises two storeys and a basement with a rectangular main house and north-eastern service wing. It is built of whitewashed rubble stone with slate-hipped low-pitched roofs with deep-bracketed eaves. The north-western wall faces the sea. The second story has six windows above which are two pediments over the and fifth bays and below which are two first-storey large round bay windows. The western wall has a two-story pedimented central bay with a twelve-pane window in its second story and a fifteen-pane window in its first storey. There are similar twelve-pane second-storey windows to the left and right of the central bay. On the first-story, to the left and right of the central bay are tented iron verandas, the left with twentieth-century glazing and the right with internal French windows. Incorporated in the eastern wall is the original service court gateway, which is arched with a bellcote over it. The bellcote retains its bell and has an octagonal spinelet.

(Sources: NMR Site Files, Cardiganshire, Domestic, SN56NW; Cadw listed buildings database; Jones and Widgey, Historic Cardiganshire Homes and their Families (Newport: 2000), p. 17)
A.N. Coward, RCAHMW, 04.05.2018