Nennius completes his chapter 37 with these words: "Thus the maid was delivered up to the king, who slept with her, and loved her exceedingly."
The story continues in chapter 38 that Hengist (the Anglo-Saxon chief) became the father-in-law to Vortigern and advised him to accept more Saxons to the island. Hengist then sent for his son and brother who brought "forty ships". Nennius states that the "incautious sovereign" agreed to this.
As if Vortigern needed more to make his name odious to the people Ninnus was writing, he adds in chapter 39: "In the meantime, Vortigern, as if desirous of adding to the evils he had already occasioned, married his own daughter, by whom he had a son." Can you believe it? Incest, the taboo of all time was added to the "evils" that Vortigern committed.
Much more of this story as given by Nennius is found in chapters 39-49. It would seem that Vortigern was advised by "his twelve wise men", to "Retire to the remote boundaries of your kingdom: there build and fortify a city to defend yourself, for the people you have received are treacherous; they are seeking to subdue you by stratagem, and even during your life, to seize upon all the countries subject to your power, how much more will they attempt, after your death!"
It is here, in the "remote boundaries" that Vortigern founds his fortify city in the mountains of Snowdonia.
For those who have an interest in reading the rest of the story, see Medieval-Sourcebook:Nennius: Historia Brittonum, 8th century.