Black's Picturesque Guide through Wales (1851) described St John's as "an ancient and finely proportioned edifice, with a noble quadrangular tower, surmounted by pierced battlements and four open gothic pinnacles...

John Newman, in his Glamorgan volume of the Pevsner Buildings of Wales series, describes the pinnacled west tower as a "magnificent marker".

[2] At a height of over 40 metres[3] the tower is in four stages, faced in grey limestone ashlar with details in buff coloured Dundry stone.

Today it still has a crown of openwork battlements, reminiscent of churches in the West Country of England, and is dated c. 1490 because of the similar Jasper Tower of Llandaff Cathedral which was built at this time.

[6] This allowed the reconstruction of St John, with extensions to the church made in 1886–1897 using carboniferous limestone quarried from Culverhouse Cross.

[7] St John's stained glass windows date from circa 1855, in the north chapel, with references to the Bute family.