St Mary's was built inside Alabum, a Roman fort, on top of Llanfair Hill.

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 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.

These include a file of information concerning wall paintings, colour transparencies and black and white postcards.

[3] In summer 2023, stone paving slabs from the historic pathway in the churchyard between the lychgate and the church's entranceway were stolen by thieves.