In the end Vortigern summoned his magicians, asked them for their opinion and ordered them to tell him what to do. They all gave him the same advice: that he should build for himself an immensely strong tower, into which he could retreat in safety if he should lose all his other fortresses. He surveyed a great umber of places in an attempt to find a site suitable for this, and in the end he came to Mount Erith. [VI.17]

SNOWDON, in the Historia "mons Erir," is the site of Vortigern's tower, the structure that leads him to search for the fatherless boy whose blood will cement the mortar. The search leads to Merlin's revelation of the two dragons beneath the tower, unleashing Merlin's prophecies. The story and location are taken from chapters 40-42 of the Historia Brittonum. Tatlock points out that Eryri is a common Welsh designation for the whole region of Snowdonia (p. 63). He also suggests that Arthur's later reference to a fight against the giant Ritho of Mount Aravia is, unknown to Geoffrey, another Snowdon reference, as there is a giant named Rhitta or Ricca associated with Snowdon (p. 64). Finally, one of Merlin's prophecies refers to Mount Aravia [VII.3]; again, Tatlock suggests that Geoffrey did not recognize that Aravius was an error for Eryri (65).

Vortigern eventually flees to what seems to be a different tower, near MONMOUTH.

There is an article about Snowdon on the Vortigern Studies site.