So, for a bit of Asul History!
The central and southern part of the Western Kingdoms - not counting the area of modern day Therion which was always a bit culturally distinct due to the relative isolation its mountain ranges afforded it - was originally home to various Celt-like tribal peoples of Humans and, in the more southern areas closer to Avari, (Wood) Elves. The Arcturian Empire, Asul's Rome equivalent, began in the south-east of modern day Arcturium and eventually ran most of the south and a bit of the central part of the Western Kingdoms, plus a few frontier settlements in other regions at various points in history - the Underdark nation of Avernus began as a mining colony for the Empire.
The empire itself was founded by Imperator Gaius Arcturus Tiberius Aurelius Augustus who was, according to legend and history, not only a Half-Dragon (Gold Dragon) but also descended from Ektimous, who was the son of a mortal man and Itonia, one of the Akhean Titans. So the various Emperors were not only Dragonblooded, but also Titanblooded.
Anyway. The Empire, like Rome, in its latter days had a lot of problems with decadence, overextension, and problems of centralizing power amidst various ambitious local lords. The Empire finally came to an implosive end, 4258 years ago, when the Emperor, Claudius Arcturus Tiberius Domitius, was assassinated by his brother Lucius and twelve co-conspirators. Lucius's bid for power was kind of a failure as he himself was then killed, and the empire fragmented. A period of endemic warfare known as the War of Kings followed, during which many modern nations like Arcturium and Lareaux formed.
The southern nations of the Western Kingdoms tend to be a bit more socially and technologically advanced, especially Arcturium, thanks to the longer history of Arcturian rule and having more of an urban and 'civilized' history. The more central countries tend to be more feudal and traditionally Medieval.
So you have nations like Arcturium which is basically Renaissaince, Lareaux which - social backwardness aside - is kind of High Medieval, and then when you go north you get the more traditionally 'Medieval Fantasy' stuff.
As for the Druidic notCelts - they were mostly assimilated into the Arcturium Empire, killed by them, killed in later times by Cuthbertines, or retreated into the deeper forests or joined up with other Druidic peoples elsewhere.
Also, consider, if the Tyrant was telling the literal truth about his memories and wasn't being poetic, then he is older than Therion by some measure as Therion is only ~2900 years old, while Lareaux is between 4258 and 4164 years old.
More Drowrambles are coming. Also Asul maps are likely coming.