"The kingdoms of the Hyborians - Aquilonia, Nemedia, Brythunia, Hyperborea, Koth, Ophir, Argos, Corinthia and the Border Kingdom - dominate the Western world." -- Robert E. Howard: "The Hyborian Age"
A loosely-knit kingdom comprised of city-states, lying east of Nemedia, south of Hyperborea and the Border Kingdom, and north of Corinthia and Zamora. Its Nemedian frontier is the north-flowing Yellow River. Another river, the Frozen, flowed eastward along the Brythunian side of the Graaskal Mountains and marked the northern border.
The highlands in the northeast were cleft by numerous passes, through which Hyrkanian invaders poured during the years following the Age of Conan. Interior Brythunia was a land of fertile, subhumid prairies interspersed with very thick, wolf-haunted coniferous forests.
Brythunia was probably primarily agricultural, with its aristocracy based upon land-owning boyars who made their headquarters in the fortified cities. The most fertile soils were probably to be found in the alluvial deposits of the interior river valleys and in the southern piedmont.
The ethnogeny of the Brythunians was complex. The people were 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 was 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 by Conan's first visit to Brythunia, the capital had moved to Pirogia. By his second visit, the capital had moved back to Sargossa, probably due to a war with Corinthia having erupted.
The lords of Brythunia were probably as thornily independent of their nominal king as were the Polish aristocrats who dwelled in the same land millennia later. 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.