Interior Brythunia is a land of fertile, subhumid prairies interspersed with very thick, wolf-haunted coniferous forests.
Brythunia is primarily agricultural, with its aristocracy based upon land-owning boyars who makes their headquarters in the fortified cities. The most fertile soils are probably found in the alluvial deposits of the interior river valleys and in the southern piedmont.
The ethnogeny of the Brythunians is complex. The people are said to be an admixture of refugees from Hyperborea the Elder and ancient Acheron, together with later Hyborian invaders and Zamorans. The absence of aboriginal genes leads to the inference that Brythunia is virtually uninhabited at the time of the first Hyborian drift to the south, which apparently thrust southwestward at the tempting target of Acheronian Nemedia, rather than cross the rugged Graaskals.
After the Hyborian conquest of Acheron, refugees from there and from Hyperborea the Elder (subjugated by Aesir) established city-states in Brythunia. As the Brythunians flourished and moved southward, they mingled with the more ancient Zamorans.
It is of note that the country appears to have two different royal capital cities, Sargossa and Pirogia. Sargossa is the original, traditional capital, but the capital was later moved to Pirogia. Some time after the move, most likely due to the war with Corinthia, the capital moved back to Sargossa.
The lords of Brythunia are thornily independent of their nominal king. According to the Saga, the women of Brythunia were mostly blonde, spirited, and beautiful, much sought by slave traders.
References: Hyborian Age I-II, Thing in the Crypt, Tower of the Elephant, Shadows in the Moonlight, Road of Eagles, Devil in Iron, Scarlet Citadel, Return of Conan, Conan the Defiant, Conan the Hunter, Conan the Amazon, Conan the Savage, et al.