The church of St Gwenllwyfo is located on the west side of a lane running south-east from Penysarn between Nebo and Traeth Dulas, to the south-west corner of the former Llys Dulas estate. It is set back from the road within an irregularly-shaped churchyard.
It was built in 1856 by Henry Kennedy, architect of Bangor, in lancet style. It is constructed of rubble masonry with gritstone dressings, including continuous sill band and raking plinth, with dressed quoins, under a slate roof with stone copings, the chancel with a decorative iron ridge. Its plan consists of a nave with four bays, two-bay chancel, three-stage west tower and spire, and a south-west gabled porch. The tower is narrow with a disproportionately tall spire.
Inside, the nave is of five roof bays with an exposed roof of arch-braced collared trusses down to wall posts on shaped corbels. A pointed arched doorway leads to the west tower vestry. The chancel is raised by one step and has closely-spaced, braced collared trusses. The chancel arch is pointed with broach stop chamfered angles and a hoodmould with facial stops; the inner arch is moulded with floriate capitals on engaged pilasters set on shaped corbels. The sanctuary is raised by three steps with a moulded sanctuary rail on cusped crossed braces and stick balusters. Fittings also include a font from the old church (NPRN 43610), retooled, consisting of an octagonal lobed bowl on a new base.
The church is notable for its fine collection of fifteenth-century and sixteenth-century Flemish stained glass panels, the second largest such collection in the world, and also its early seventeenth century brass memorial plaque.

Sources: extracts from Cadw Listing database
Haslam et al., Buildings of Wales: Gwynedd (2009), p.123-4

RCAHMW, 18 November 2014