Llanddwyn island is rich in legends, in particular in its association with Dwynwen. The name Llanddwyn means 'The church of St. Dwynwen'. Dwynwen is the Welsh patron saint of lovers, making her the Welsh equivalent of St. Valentine. The island bears the ruined remains of St Dwynwen's Church (Eglwys Santes Dwynwen). Unusually for churches in that region it has a cruciform floorplan.
Archaeologists examined the ruins of St Dwynwen's in 2011 and again in 2021. The 2021 dig discovered traces of even older buildings below the ruins.
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.