This one is a bit more detailed.
All of this is guesswork by scholars, of course. It’s called the Dark Ages for a reason.
Here’s what I made up for the book – a summary from the marketing packet my publisher is making me write:
Jack’s world is essentially Arthurian England, circa 500 ce. The events in Beanstalk and Beyond take place in 496 ce, though few of the characters involved would be certain of the actual Roman year.
Camelot is the military headquarters of a group of Romano-British cavalry who begin referring to themselves as the Far Western Roman Empire, as it appears the regular Western Roman Empire is in the process of collapse. In 496, Aurelius Ambrosius, the last true King of the Britons, remains on the throne. His rule is safeguarded by General Artorius Pendragon, whose knights have been clearing away Saxons by the horde because they understand how stirrups work .
Arthurian Scholar Geoffrey Ashe, one of my main sources, has long contended that there is ample evidence that around the turn of the 6th century, the Romanized Britons managed to stall the Saxon encroachment for a decade or so, by virtue of a successful military commander “that we have no reason not to call King Arthur”.
While Artorius is referred to, he does not appear in Beanstalk. His knight Bedwyr (ne Bedevire) appears, as does a character who may or may not be the Fisher King (Josephus), Abrosius’ crown Prince Charein (aka Prince Charming) and the legendary Bard Taliesin all appear directly, and influence events.
It should be noted that despite the efforts of the Saxons and the Britons, most of the Isles are still governed by a patchwork of tribal kingdoms.
While the magic in this world is quite real, it is also fading. This is referenced a few times in Beanstalk, though the reason behind the fade is never explored. The short version is that the Atlanteans over-used it past the point where it would renew itself. The premise behind magic is the same as that in GAFT (explained here is a bit more detail), even though most of the practitioners in Jack’s time know very little about such things.
Which is interesting in its’ own right.