Historical Fiction
Mary Stewart riffs off of early medieval "historical" chronicles, like the ones written by Nennius (8th century), Gildas (6th century), and Geoffrey of Monmouth (11th century).
Each of these dudes wrote about the military conquests that built the kingdom of Britain. More specifically, they wrote about the rise of King Arthur.
So here's the thing. When we talk about historical fiction, we usually mean that an author is setting his or her story during a specific time period, or setting his or her characters loose during historical events we know to have been real.
There's some of that going on in Stewart's book. We know that Vortigern was a real guy, and that Hengist and Horsa—though their names seem unreal—really did a lot of damage to Britain. But she's also working with characters who may or may not have been real, or who may have been a combo of several historical figures. Ambrosius is one of these dudes.
And then there are the straight-up legendary characters. Merlin and Arthur fall into this category. By using these ancient characters, Stewart isn't using history to make a new story; she's actually doing some myth-making of her own.
So in addition to writing a novel, Stewart is also walking in the footsteps of Geoffrey and Gildas and Nennius, using history and legend to make sense of the past. We think that's a pretty awesome twist to the historical fiction shtick.