Interior
The lower part of Derwydd is of exceptional interior interest, containing, in particular, three Jacobean fireplaces of vigorous West Country type.
Direct entrance to the hall, without porch. Double arch to left, screening stairs and service rooms. The hall has a large chimney at the centre of the east side, with a large window recess at left and a former door lobby at right, the latter entered by a stone door arch. Large exposed transverse beams, with a moulded cove to the ceiling panels between. The room is dominated by the large fireplace with its segmental stone bressummer and the fine overmantel of 1644: this displays the Vaughan arms on an oval, upon a large cartouche the edges of which break into scrolls, and on which there is a seraph at top with crossed wings and another below with wings spread. Supporters each side as female griffons the lower parts of which are foliage. Terms each side as pilasters, with fruit in their hair; enriched cornice.
Library in the original north wing: coffered plaster ceiling in four square panels with copious floral enrichment; guilloche ornament to the beam soffits; cornice with egg and dart and an enriched cyma. Lozenge dividers in the panels with central roundels and features. This room also dominated by its fireplace: a fine Jacobean surround and overmantel, partly restored, in dark wood. Bressummer on volute-capped pilasters; the bressummer ornamented with eight circles in a chain with smaller circles; chamfer with run-out stops. Deep shelf with carved edge. Two-stage overmantel: central picture in circle between two panels with arched headed tops; upper stage with four lozenges.
Above the library is the room known as 'the King's Room', otherwise the Drawing Room. Late C17 ceiling and fireplace. Deep frieze and cornice to the ceiling with cartouches and heraldry at intervals, with linking floral trails and little modillions under the cornice. The ceiling itself is plain apart from a central roundel with sprigs of foliage radiating from a central pendant; four features outside the roundel. Fireplace with a thin shelf on thin pilasters; above is an oblong panel with a bayleaf oval, a frieze similar to that of the ceiling, and a rounded pediment with central arms on a cartouche and swags held by putti above the pediment. Rustic figure each side standing on a plinth and within a cove.
The larger bedroom at the south end of the older part of Derwydd is entered by a Jacobean door with an arch-headed top panel above the lock rail and two panels below. This room is plain apart from a ceiling with large coffers, the sides of the beams moulded but not enriched, and a fine Jacobean fireplace. Plain shelf on a plain bressummer, apart from two heads in relief; plain pilasters apart from fluting. Above the shelf is a figure wearing a tunic and sash, with cross bands, and carrying a sword, in a plain niche; to left and right are ramped broken volutes with swags of foliage and fruit carried from the top volute eyes. Pilasters each side with inverted bay branches; enriched entablature above with pulvinated frieze.
In the corridor between the latter room and the King's Room are two late C17 doors with broad panels. In the older part of the house there is exposed roof timbering.
A fine feature of the 1888 alterations to Derwydd is the Tudor style staircase. This rises from the inner hall, the space behind the arcade at the north of the main hall. At the head of the main flight is a quarter landing, with a lesser flight ahead to the upper hall on the first floor and a side flight to the corridor of the older part of the house. A separate staircase of similar detailed design commences in the upper hall and in three flights with quarter landings rises to the upper storey of the newer part of the house. Dark hardwood staircases with moulded handrails swept to meet newels; turned balusters; carved newels with turned finials; broad strings with quatrefoil sinkings on the outer face; triple curtail steps. Integral with this design is panelling to staircase dadoes and cupboards, in similar wood, arcading on the upper floor, and stone arcading in Perpendicular style forming a side lobby to the lower staircase.