St Mary's Church was built inside Alabum, a Roman fort, on top of Llanfair Hill. A monastic cell from around 1126 was already on the site before building commenced. It is one of the largest medieval churches in the county, and has a substantial tower at the west end with a square stair-tower on its northeastern corner. The church is built of rubble stone with plain tiles on the roof. Earlier dressings are mostly in red sandstone, while nineteenth century dressings are in Bath stone. The nave and chancel are directly connected to the tower, to the porch on the south side and to the vestry and organ chamber on the north. The hymn writer William Williams Pantycelyn (1717–1791) is buried in the churchyard, where there is a memorial to him.
References:Saint-Georges de Boscherville Abbey is a former Benedictine abbey. It was founded in about 1113 by Guillaume de Tancarville on the site of an earlier establishment of secular canons and settled by monks from the Abbey of Saint-Evroul. The abbey church made of Caumont stone was erected from 1113 to 1140. The Norman builders aimed to have very well-lit naves and they did this by means of tall, large windows, initially made possible by a wooden ceiling, which prevented uplift, although this was replaced by a Gothic vault in the 13th century. The chapter room was built after the abbey church and dates from the last quarter of the 12th century.
The arrival of the Maurist monks in 1659, after the disasters of the Wars of Religion, helped to get the abbey back on a firmer spiritual, architectural and economic footing. They erected a large monastic building one wing of which fitted tightly around the chapter house (which was otherwise left as it was).