"It found Bendigeidfran in Caer Saint in Arfon"
Caernarfon Castle at night
Creative Commons AttributionCaernarfon Castle at night - Credit: Richard Outram

 Arfon was one of the Welsh medieval cantrefs (see bookmark for page 3). It was situated in the northwestern part of Wales, opposite the Isle of Anglesey. The name is still used for one of the administrative areas of the county of Gwynedd.

Caer Saint is an old name for the town of Caernarfon in present-day Gwynedd. Caer means fort, and Saint is a version of Seiont, the river which enters the Menai Strait at Caernarfon. During the Roman occupation of Britain Caernarfon was the site of a Roman fort called Segontium, and later Edward I built a castle there. In the story 'The Dream of the Emperor Maxen', an alternative name for Caernarfon is Aber Saint ('the mouth of the River Seiont').