Vortigern could have ruled Britain during the early to mid 400s, having grabbed the British throne from Constantine, the father of Ambrosius Aurelianus and Uther Pendragon. Vortigern then exiled Ambrosius and Uther (who were then just children) to Brittany to live with their uncle. Vortigern then proceeded to invite the Saxons, under Hengist and his brother Horsa, into Britain as mercenaries to help fight the invading Scots and Picts. Vortigern became smitten by Hengist's daughter Rowena and in return for marrying her, he gave the Saxons what is roughly the present county of Kent to live in and rule themselves. The Saxons used this as a base to increase their presence in Southern England. And even when Ambrosius eventually killed Vortigern and took over the British throne, Saxon menace remanined.
Vortigern is supposed to have retreated to Dinas Emrys when the Saxons began to really threaten him. The story of his building this fortress is first told by Nennius and then by Geoffrey of Monmouth. Vortigen had difficulty in actually getting his fort to stay up, and consulted his wise men who said "You must find a child born without a father, put him to death". Vortigen's envoys found such a child, who was prepared for sacrifice. However the boy found the true cause of the building subsiding, an underground pool that was causing the foundations to crumble. Digging opened up a pool in which there were two dragons, one red and one white, who were locked in mortal combat. The red dragon eventually won, and the boy explained to Vortigen the meaning it. The White Dragon represented the Saxons. The Red Dragon represented the people of Britain .
Nennius, writing around the year 800, still saw the possibility of expelling the Saxons. And hence has the Red Dragon, the British, winning the fight.
Geoffrey of Monmouth's story, the fighting is interpreted differently. Here we have a post-Norman writer who had apparently given up all hope of British self-rule. And it is the Red Dragon who loses. Eventually he has Vortigern being burned to death at Ganarew. When Ambrosius returns from exile in Brittany to reclaim his throne, he reaches Vortigern's stronghold, and burns it to the ground with Vortigern inside.
Nennius names the mysterious boy Ambrosius ("Dinas Emrys" means "Fort of Ambrose"), while Geoffrey says that Merlin was the boy. Vortigern's men found Merlin outside the gates of Carmarthen, which Geoffrey gives ruined building at Dinas Emrys as "Kaermerdin."
The Eliseg Pillar traces the lineage of Eliseg, a Welsh ruler, on a monument that is more than 1100 years old. The Latin inscription, though illegible in areas, traces his lineage back to Vortigern. And this can be used to place Vortigen at around 400
In early Welsh texts, the name Vortigern is "Gwrtheyrn." The name Nant Gwrtheyrn, then, means "Vortigern's Hollow." His grave is said to be in this area by a stream that runs from the hills down to Caernarvon Bay.
Nennius lists "Cair Guorthigirn" as one of his twenty-eight cities of Britain, but as is the norm, there is debate as to where it actually was. Was it Dinas Emrys in Snowdonia, or perhaps Little Doward just north of the town of Monmouth and overlooking the Wye River.
So if Vortigern did rule Britain, there are a number of possibilities for his stronghold.