1) The name of A.A. matches perfectly that of the fourth century Governor of Gaul (whose territories included those of Britain) and his famous son, St. Ambrose. Vortigern's grandfather Vitalinus is said to have fought A.A. at Wallop in Hampshire. Such a battle reference puts A.A. well before Vortigern and negates the possibility that A.A. was a boy during Vortigern's reign.
2) St. Ambrose and his father lived at Trier on the Moselle. The Campus Elleti in Wales where Vortigern's men are said to have found the boy A.A. comes from a Welsh place-name Maes Elei and this is plainly a substitute for the Moselle (Mosella/Mosellae).
3) Dinas Emrys is a relocation for Amesbury, the latter thought (wrongly) to contain the name of Ambrosius. Dinas Emrys was placed in Eryri because this mountain range was fancifully connected to the Welsh word for eagle, and both St. Ambrose and Magnus the Tyrant (easily confused with Vor-tigern) are known to have been at Aquileia, a place-name that could have been incorrectly linked with the Latin word for eagle.
4) Trier was in Gallia Belgica, 'Gaul of the Belgae', and A.A.'s Wallop in Hampshire was in the ancient tribal territory of the British Belgae. Gallia could be used in medieval sources for both Gaul and Wales.
5) A.A. is said to have been given Dinas Emrys and the western kingdoms of Britain by Vortigern. This is impossible, as Gwynedd belonged to Cunedda and his sons. Obviously, a mistake has been made here for Amesbury, which was inside of what was to become Wessex, the kingdom of the West Saxons.
6) A.A appears to have been identified in folk belief with the god Lleu, styled Lord of Gwynedd,who was himself identified by the Welsh with the god Mabon. The Campus Elleti ballgame story is paralleled in the Irish story of Mac Og, the 'Young Son', the Gaelic version of Mabon.
7) A.A. was further identified with Merlin (Myrddin), himself possibly a form of Lleu or an avatar of that god. Geoffrey of Monmouth places Merlin at the springs of Galabes, his transparent attempt at the Guoloph/Wallop of the hero Ambrosius
In conclusion, the Ambrosius Aurelianus who first appears in the pages of Gildas is a purely legendary figure, based on the known historical Ambrosii of the Continent. He was mistakenly transferred to Britain during the normal course of folklore development, largely due to a confusion of place-names. There is no reason to believe that either Ambrosius - father or son - ever set foot on British soil. To concoct some famous war-leader of the Britons who happened to have been named after one of the Ambrosii is to ignore points 1-7 above.