"The proudest kingdom of the world was Aquilonia, reigning supreme in the dreaming west." -- The Nemedian Chronicles
The foremost of the Hyborean kingdoms during Conan's time, situated east of the Pictish wilderness, west of Nemedia, north of Zingara and Ophir, and south of Cimmeria.
Aquilonia apparently had a pleasant, temperate climate rather like that of modern France or Germany. The Border Range of Nemedia naturally delineated its borders on the east, the thickly forested hills of Cimmeria to the north, the large Thunder River to the west, and the Tybor River in the south. Another area of rugged mountains arose in the southern province of Poitain, which was flanked by the Alimane and Khorotas Rivers.
The frontier region of Westermarck between the Thunder and Black Rivers was thinly settled and beleaguered by Picts. Certain barons sponsored its provinces of Oriskonie, Conawaga, and Schohira. Thandara was independent. East of the Thunder River and curving northward toward the Cimmerian lands lay the narrow buffer province called the Bossonian Marches, or Bossonia. The nobles of Aquilonia were apparently not able to extend their sway over the staunch villagers of this region.
Another enclave that resisted the power of Aquilonian nobility was Gunderland in the far northwestern hills. Its ancient people, of the purest Hyborean stock, "never considered themselves exactly Aquilonians" (according to Howard).
Among the other provinces or counties mentioned in the Saga were Pellia (the only region ruled by a prince), Tauran, Couthen, Manara, Thune, Raman, and Karaban. Baronies mentioned include Kormon, Torh, Amilius, Lor, Imius, and Attalus.
The capital of Aquilonia was Tarantia, lying on the Road of Kings, the main east-west trade route of the Hyborian world. Interior Aquilonia was well drained by the rivers Shirki, Khorotas, Ossar, Fury, and the presumed Valkia. East of the Shirki and extending nearly to the foothills of the Border Range of Nemedia lay fertile plains interspersed with "tame" woodlands.
Before Conan became king, much of the good cropland was sequestered by the aristocracy for use as hunting preserves, forcing the expanding population into risky colonizing ventures in Westermarck.
The history of Aquilonia is complex and can only be adequately appreciated through a reading of the Saga itself. We have assumed, after considering both historical and geographical factors, that Aquilonian Acheron fell to invading Hybori some time after Nemedian Acheron. With the establishment of a Hyborian kingdom in Aquilonia, the nation became "supreme in the western world", the most powerful from a commercial and military standpoint -- though perhaps second to Nemedia in cultural richness.
By Conan's time, Aquilonia's royal house was in a state of decadence and the Westermarck region on the verge of revolt. Conan first came to power in the Westermarck. Subsequently, a Poitainian faction helped Conan usurp the Aquilonian throne. He ruled for some 20 turbulent years before abdicating in favor of his son, Conan II, called Conn.
Years later when Aquilonia was at its zenith, it annexed Zingara, Argos, Ophir, and western Shem. Some 500 years after Conan's time, internal decay and a Pictish invasion destroyed Aquilonia.
References: Hyborian Age I-II, Tower of the Elephant, Hall of the Dead, Queen of the Black Coast, Red Nails, Beyond Black River, Treasure of Tranicos, Wolves Beyond the Border, Hour of the Dragon, Conan of the Isles, Conan the Liberator, Conan the Great, Conan the Rogue, Conan and the Death Lord of Thanza, et al.