St Mary's church site is located in close proximity to the castle mound (NPRN 303778) some 45m to the north-east. The church site is 500m south of Ffynnon Fair (a farm name but also the site of a spring) and some 700m west of St Cynllo's Church (NPRN 400353). The stream flowing from Ffynnon Fair runs through a wood, Allt y Ffynnon, which lies roughly midway between the two church sites. Historic (1898) Ordnance Survey mapping also depicts the site as Llanfair-Treflygen. The church is a Scheduled Ancient Monument. Said to have fallen out of use about 1800, it is now in ruins. Fragments of the east & west walls remain, suggesting a single cell structure measuring 10m east-west by 5m. The structure is set within a subrectangular enclosure measuring 32m east-west by 30m, defined by scarps some 1.1m high.

RCAHMW, 20 May 2014