St Tanwg's Church is situated within sand dunes on the foreshore, some 20m from the high water mark. It is medieval in origin, and houses three fifth to sixth century inscribed stones and two cross incised stones. The first, once part of a much larger slab, is now built into the south window of the chancel. Its inscription has been interpreted as 'Equester(s) lies here'. The second was used as a lintel in the east window in the nineteenth century, but presently lies loose in the church. Its inscription is thought to read 'The stone of Engenuus Iarrus(?). He lies here'. In 2008 another inscribed stone and cross-incised stone were discovered during the levelling of a courtyard outside the west door. The inscribed stone measures 1.59m length and its inscription has been interpreted as 'The stone of Gerontius, here he lies, the son of spectatus'. The cross incised stone measures 0.94m in height and has a cruciform head and shaft with a cross within a lozenge on one face- with stones at St David’s Church, Llanllawer (NPRN 308778), St Tecwyn's Church, Llandecwyn (NPRN 43903) and St Sulien's Church, Silian (NPRN 402554) it is one of only four definite examples of this pattern in Wales. Most of the cross's left arm has been removed. This combination of elements of a free-standing cross with those of an incised cross is rare in Wales. Another cross incised stone, dated to the seventh to ninth century, has been re-set above the door in the external wall of the west gable. St Peter's, Llanbedr (NPRN 43897) is known to have once been dependant on St Tanwg's. The distribution pattern of groups of churches along the seaboard of Ardudwy is of note, the churches in each group being in close proximity to one another. St Tanwg's, along with St Mary's, Llanfair (NPRN 43887), and St Peter's, Llanbedr, are one such group, being located within about a mile and a half of each other. It has recently been suggested that the group may also include St Michaels, Llanfihangel-y-traethau (NPRN 43894) and St Tecwyn's, Llandecwyn.
Much of the former churchyard is buried beneath dunes. Aerial reconnaissance in May 2005 identified a curvilinear ditch to the immediate east of church. This may delineate the former boundary of the churchyard enclosure (but may relate to a stream or watercourse depicted on historic (1889) Ordnance Survey mapping). The churchyard is currently entered through a modern (1955) lych gate located on its eastern side. In 1906 dimensions of the churchyard were given as 142 ft length x 96 ft width. It is no longer used for burial. Beneath the external east window is the grave of renowned Welsh poet and minstrel, Sion Phillips, who drowned crossing from Pwllheli to Llandanwg in 1620.
The church is a Grade II listed building. It is constructed of rubble stone and consists of a continuous nave and chancel with a small west bellcote. Measurments are given as 57ft length x 23ft breadth. The nave is thought to be thirteenth century, and has a blocked south door of that period. The chancel was extended to the east in the early fifteenth century, and featured an unusually tall east window. It retains its original roof structure, including three eastern arch-braced collar beam roof trusses. They were later ceiled with wood boards, on which wall paintings were recorded. These included the figures of saints and of the four evangelists. A sketch shows the symbols of the evangelists and an opposed angel and cheerful devil. Other traces of wall painting include undecipherable black lettering on the eastern end of the southern wall and borders and other traces on the north wall. The lower border appears to have included a now lost inscription and splashes of colour. An oak rood screen was installed, probably in the fifteenth century. Its top beam remains in place and there are some remains of a later western gallery. Two windows at the east ends of the north and south walls are thought to date to the fifteenth century. The cross incised stone in the west gable bares the inscription 'REP 1685', probably the date of a seventeenth century phase of alterations. The tall east window was replaced with a smaller window. Windows were also inserted in the centre of the south and north walls. It is thought that a 4m stretch of the centre of the north wall also dates to this time. The building was abandoned following the construction of a new church, St Tanwg (NPRN 43902), at Harlech in 1841. The fifteenth century octagonal font was removed to the new church at this time. The building was restored and re-roofed in 1884. The present west door dates from this time.
Beverley Smith, J, Beverley Smith, Ll, 2001, History of Merioneth II, 343-4
Davidson, A, 2008, ‘Two new early medieval stones from Llandanwg’, Archaeology in Wales 48, 73-75
Gwynedd Archaeological Trust, 2000, Historic churches of Gwynedd: gazetteer, 391
Ordnance Survey, 1889, 1st edition 25inch
N Vousden, RCAHMW, May 2012