The parish church of St Cybi stands at the centre of the Roman fort that gives Holyhead its Welsh name, Caergybi. The current building is mainly 13th–16th century, replacing a much earlier monastic foundation of which little visible trace remains. The south porch is fine Perpendicular work, and inside, the church has a beautiful set of late-Victorian stained-glass windows by William Morris's firm to the designs of Edward Burne-Jones.
Next to the main church, in one corner of the Roman compound, is the tiny 14th-century chapel of Eglwys y Bedd ("the church of the grave"), traditionally identified as the burial place of an Irish chieftain killed by St Cybi. It is among the smallest medieval chapels in Wales — a single room, barely more than four metres across.