Large hamlet near Bontnewydd, Caernarfon. Caernarfon, 2002 Caernarfon (the Welsh spelling is now normally used in preference over the Anglicised form, Caernarvon or Carnarvon) is a Royal Town in Gwynedd in north-west Wales. ...
Altogether, I sense that the element of the king that consults sorcerers against a victorious innocent (holy man/saint/boy) is as foreign to Dinas Emrys as is Ambrosius Aurelianus, and I opt for the explanation that it was a wandering story that attached itself to the core, already present.
An alternative to this theory, though based on the same principle of mis-interpretation, is the connection between Vortigern and Dinas Emrys as based on a mis-representation of the name of the fortress.
Maximus was closely connected to Gwynedd, for the Roman unit based at Segontium (Holyhead) accompanied his army to the continent.