"And they saw Aber Saint, and the castle at the mouth of the river"
The estuary of the River Seiont, looking towards Anglesey
Creative Commons Attribution Share AlikeThe estuary of the River Seiont, looking towards Anglesey - Credit: Eric Jones

Aber Saint is the mouth of the River Seiont, which reaches the Menai Strait at Caernarfon. As mentioned in the bookmark for page 28, Caernarfon is also referred to in The Mabinogion as Caer Saint (‘The Fort of the Seiont’). The ‘castle' referred to in the text is not the one to be found in present-day Caernarfon,  which was  built by Edward I in the 13th century.