Gwydir Castle is an irregularly planned house in late Perpendicular Gothic style with some late 16th century Renaissance detailing. It is of roughly J-shaped plan, constructed of slate-stone blocks with sandstone dressings and slate roofs. Many of the building materials came from Maenan Abbey.

The UpperHall, known as the Hall of Meredith, has three main bays, then a half-bay at the south-west, which indicates a dais canopy. The ceiling was scraped of plaster, including a flat Elizabethan ceiling, in the 1950s, resulting in the reincorporation of the open roof into the hall. The fireplace at the south-west end has a simple timber lintel, probably meant for plaster. At the north-east end is the oak-framed smoke-bay. The roof looks early as it is shallow-pitched with arched-braced collar-trusses with apex struts, and short cusped wind-braces. It is possibly fifteenth century.

The four storey solar tower is at right angles to the Hall of Meredith and is early to mid sixteenth century in date. The Tudor archway is dated 1555 and had a timber-framed upper storey until 1820. There are also north and west wings, the latter being a nineteenth century copy of the mid-sixteenth century former. The building was gutted by fire in 1912 and 1922. There is a painted stencilled frieze above panelling and painted genealogical table in the dining room.

Sources include:
Cadw, Listed Buildings Database
Haslam, Orbach and Voelcker (2009), The Buildings of Wales: Gwynedd. Pevsner Architectural Guide, page 374, 378.

RCAHMW, October 2009.