Asgard is the celestial realm of the Aesir, the principal family of Norse gods, and functions in the cosmology as the fortified upper world, home of Odin, Thor, Frigg, and their kin, and containing Odin's hall Valhalla, where the chosen slain feast in preparation for Ragnarok. It is joined to Midgard, the world of humans, by the flaming rainbow bridge Bifrost, guarded by the ever-watchful Heimdall.
The Eddas describe its building: the gods hired a giant mason to raise its great wall, and only the trickster Loki's intervention (transforming himself into a mare to lure away the giant's indispensable stallion) cheated the builder of his payment, one of many stories in which Asgard's security is bound up with the gods' uneasy dealings with the giants who threaten it. For Asgard is not eternal: the same sources that describe its glory foretell its fall at Ragnarok, when the giants and the forces of chaos storm across the broken Bifrost and the gods go down fighting. Asgard thus embodies the Norse sense of the divine order as something splendid but besieged, defended by heroism against a doom already foreseen.