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:The Church of St Donatus name refers to Donatus of Zadar, who began construction on this church in the 9th century and ended it on the northeastern part of the Roman forum. It is the largest Pre-Romanesque building in Croatia.
The beginning of the building of the church was placed to the second half of the 8th century, and it is supposed to have been completed in the 9th century. The Zadar bishop and diplomat Donat (8th and 9th centuries) is credited with the building of the church. He led the representations of the Dalmatian cities to Constantinople and Charles the Great, which is why this church bears slight resemblance to Charlemagne's court chapels, especially the one in Aachen, and also to the Basilica of San Vitale in Ravenna. It belongs to the Pre-Romanesque architectural period.
The circular church, formerly domed, is 27 m high and is characterised by simplicity and technical primitivism.