A NOTE ON CAER ARIANRHOD

Caer Arianrhod is another important locator in 'Math Son of Mathonwy.' It is within both walking and boating distance of Caer Dathal (Caer Engan) and Dinas Dinlle, the 'Town of the Fort of Lleu.'

Traditionally, Caer Arianrhod has been identified with a coastal rock between Dinas Dinlle and Maen Dylan, the 'Stone of Dylan.'  Dylan was the god Lleu's twin brother, another son of Arianrhod.

I think, however, the real Caer Arianrhod was an entirely different place.  

The Bryn Arien mentioned as lying on the way to Caer Arianrhod along the sea from Dinas Dinlle has been identified with a hill near Brynaerau (see Sir Ifor Williams, Pedeir Keinc y Mabinogi , Cardiff, Second Edition, 1951, pp. 278-9).  Cefn Cludno or Cefn Clun Tyno was thought by Sir Ifor Williams to be in Capel Uchaf and others have since agreed (see 150 Jahre "Mabinogion" - deutsch-walisische Kulturbeziehungen by Bernhard Maier and Stefan Zimmer, Walter de Gruyter, 2015).  There is a Coed Tyno just a little north of Capel Uchaf. 

Foel Hillfort near Tyno

Foel Hillfort

(https://coflein.gov.uk/en/site/95338/images)

If Gwydion and Lleu really did take horses from Coed Tyno, then the most obvious fort close by is that of Foel.  

However, there is a Bryngwydion, 'Hill of Gwydion', just a little north of the Craig-Y-Dinas promontory fort.  And as Dylan's Stone is near the mouth of the Afon Llyfni, the 'port' of Arianrhod must be at the mouth of the river.  Note also the Lleuer (= W. lleuar, 'light, brightness'?; see https://www.academia.edu/35985785/Some_Cornish_place_names_with_lyw, citing Enwau lleoedd sir Gaernarfon by John Lloyd-Jones, Gwasg Prifysgol Cymru, 1928, for lleu-erw: lleu 'goleu') place-names at Craig-Y-Dinas, which may well hearken back to the god Lleu. The Llyfni flows through Lleu's Nantlle, and Dolbebin in Nantlle was named for the father of Math's virgin footholder, Goewin.

Craig-Y-Dinas

(https://coflein.gov.uk/en/site/93534/)

Either of these forts could be Caer Arianrhod.  The deciding factor might be that when Gwydion and Lleu take horses from Coed Tyno, they are not said to have crossed or forded the Llyfni.  To have gone to Brynaerau, thence to Coed Tyno and on to Craig-Y-Dinas would mean they were doubling back on their course needlessly. The mouth of the Llyfni could serve just as well as the 'port' of the Foel hillfort.  Finally, the two Lleuer places are south of the Llyfni, so more accessible from Foel.  Craig-Y-Dinas is on the north side of the river.  The hill of Foel is over 220 meters high, making it much more imposing in the landscape than Craig-Y-Dinas.

For these reasons, I think we must favor Foel as Caer Arianrhod.  

Rachel Browich and others have favored Aran- for the first component of Arianrhod's name.  If such scholars are correct, it might be that Aranrot was originally a name for the hill of Foel.

The following discussion of the meaning of her name is from the TRIADS (p. 284):

The standard line of reasoning for the etymology of Aran is set forth by Dr. Richard Coates in https://www.snsbi.org.uk/Nomina_articles/Nomina_35_Coates.pdf.  

However, John Koch (in CELTIC CULTURE: A HISTORICAL ENCYCOLPEDIA) is surely right in settling for Arianrhod as the correct spelling of the name: