Mae St Gwynno yn eglwys ganoloesol ar safle hynafol. Mae dwy groes garreg gynnar yn rhan o adeiladwaith y wal ddeheuol, ac yn y fynwent, mae bedd y rhedwr cyflym enwog, Guto Nyth Brân.
A medieval hilltop church with a very strange shape, two naves side by side, with the chancel running across them. This must have been the plan for the original stone church, because the arches between the two naves are early thirteenth century. But there was a church before this, probably made of timber and wattle. The church is dedicated to an early Irish saint, Sannan, and there is a holy well dedicated to him just down the hill, on the road leading to the Rock. We don’t know much about Sannan, but according to legend he was an Irish abbot and missionary and a close friend of St David. There are other churches dedicated to him at Llansannan in Denbighshire and Llantrisant on Anglesey.
In the chancel is a medieval cupboard with the ‘Arma Christi’, the hammer, nails, spear, whip and crown of thorns from the Crucifixion story and the Five Wounds of Christ. This may originally have been an Easter Sepulchre, used in the dramatic retelling of the Crucifixion story every Easter. It was remade into a cupboard and called the Bread Chest. This might have held the bread given to the poor as part of the Lewis Charity. You can see a plaque describing this charity in the chancel.
The church has been whitewashed to protect the stone. This is how the church would have looked in the Middle Ages. One writer said that Wales ’glittered with lime washed churche like the firmament with stars’.
In the churchyard, you can still see the steps and the shaft of the medieval churchyard cross. This area was a centre for the coal and iron industry, and you can see several cast-iron gravestones in the churchyard.