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The origins of the Cimmerians stretch back to the Thurian Age. The Cimmerians are the descendants of colonists from Atlantis. Living on the main Thurian continent, the colonists survived the great cataclysm which submerged Atlantis and destroyed most of the Thurian civilizations. The survivors, at this point reduced to a stone-age level of sophistication, eventually found themselves locked in multigenerational warfare with survivors of a Pictish colony. This prolonged conflict caused the Atlanteans to further devolve into little more than ape-men. With no memory of their history or even of language and civilization itself, these beings eventually redeveloped into a people known as Cimmerians.
In his essay The Hyborian Age, Howard states that "the Gaels, ancestors of the Irish and Highland Scotch, descended from pure-blooded Cimmerian clans," and he names Conan with a Celtic name, and portrays him cursing by Celtic gods, so obviously there is meant to be a strong cultural connection between his Cimmerians and the historical Gaelic people.
Cimmeria was undoubtedly a rugged wilderness, hilly, probably mountainous, heavily forested, and often cold and overcast. Based on the Hyborian map as it overlaps a modern map of Europe, there was likely a mountain range along Cimmeria's western border. Howard confirms this in The Hyborian Age: "...the ocean flowed around the mountains of western Cimmeria to form the North Sea; these mountains became the islands later known as England, Scotland and Ireland..."
According to Howard, "the Cimmerians are tall and powerful, with dark hair and blue or grey eyes". They are a proto-Celtic or proto-Gaelic people. From the evidence in the stories, they appear to be partly or completely tribal hunter-gatherers, with no central government.
Although they know of the craft of iron-working (Conan's father was a blacksmith), they still seem to have a nearly stone-age society, as Conan himself remarks: "I saw myself in a pantherskin loin-clout, throwing my spear at the mountain beasts." 
The Cimmerians are a people forged by the very harsh conditions of their land. Howard often makes reference to the Cimmerian people's hardiness and martial prowess, as well as to their many other impressive skills. They can climb seemingly unscalable cliffs, track humans and animals with ease, and stalk their prey without making a sound.
The apparent primitiveness of the Cimmerians and their sense of justice (see Noble Savage) is often juxtaposed with the corruption of the "civilized" races in Hyboria.