|Could this be the place mentioned in 'Math Son of Mathonwy' in the Mabinogion?
Then they went towards Dinas Dinllev, and there he brought up Llew Llaw Gyffes, until he could manage any horse, and he was perfect in features, and strength, and stature. And then Gwydion saw that he languished through the want of horses and arms. And he called him unto him. "Ah, youth," said he, "we will go to-morrow on an errand together. Be therefore more cheerful than thou art." "That I will," said the youth.The notes of Lady Guest's translation imply she thought so:
Next morning, at the dawn of day, they arose. And they took way along the sea coast, up towards Bryn Aryen. And at the top of Cevn Clydno they equipped themselves with horses, and went towards the Castle of Arianrod.
"DINLLEV*: DINAS DINLLE is situated on the sea-shore, about three miles southward from Caernarvon, in the parish of Llantwrawg, on the confines of a large tract of land, called Morva Dinlleu. The remains of the fortress consist of a large circular mount, well defended by earthen ramparts and deep fosses."
*Probably 'Dinlleu' with a u, not a v? to tie in with Lleu Llaw Gyffes?
She also adds: "The Rev. P. B. Williams, in his "Tourist's Guide through Caernarvonshire," speaking of Clynnog in that county, says: "There is a tradition that an ancient British town, situated near this place, called Caer Arianrhod, was swallowed up by the sea, the ruins of which, it is said, are still visible during neap tides, and in fine weather."
Indeed, there is a stack off the coast (no doubt visible from Dinas Dinlle?) called Caer Arianrhod.
You can read the story courtesy of the brilliant Sacred Texts Archive:
Posted by Rhiannon
23rd December 2005ce
Edited 23rd December 2005ce