The Kojiki, compiled in 712 CE, describes Takamagahara as the world above, reached from Ashihara no Nakatsukuni, the Central Land of Reed Plains, by the Ame-no-ukihashi, the Floating Bridge of Heaven. The bridge is not a passage anyone crosses casually. It is the route by which Izanagi and Izanami stood to stir the primordial sea and create the first island, and it is the route by which Ninigi, grandson of Amaterasu, later descends carrying the three imperial regalia, the mirror, the sword and the jewel, to found the line of rule on earth. The descent, tenson korin, is the myth on which the classical claim of unbroken imperial descent from the sun goddess rests.
Takamagahara is populated by the amatsukami, the heavenly kami, as against the kunitsukami, the earthly kami already present in the land the heavenly line comes down to govern. That distinction runs through the whole mythology: shrines, lineages and rites can be sorted, in the classical texts at least, by which register of kami they answer to. Amaterasu's own residence there, a hall with a sacred loom on which garments are woven for the kami, is the setting for the myth for which Takamagahara is best known outside specialist reading: her brother Susanoo's escalating violence there, culminating in his hurling a flayed horse through the roof of the weaving hall, after which Amaterasu withdraws into a cave and hides the sun.