Bifrost

Snorri Sturluson's Prose Edda describes Bifrost as a bridge built with great skill and craft, stronger and more cunningly made than any other structure, and identifies its familiar rainbow bands with fire: the reddish color visible in it, Snorri explains, is burning flame, placed there deliberately so that frost giants and mountain giants attempting to climb into Asgard would be turned back at the crossing.

The Poetic Edda, in the poem Grimnismal, has Odin describe the same bridge burning with flame and name it among the sacred places the gods ride to each day, since Asgard and the well of Urd beneath the world tree Yggdrasil, where the gods hold their daily council, sit on opposite sides of it. Only the gods and, in some readings, particular favored dead are said to cross it freely; Thor is noted as an exception who wades through the rivers below rather than risk the bridge's heat on his approach, which some sources connect to his elemental nature as a thunder and storm god rather than to any weakness in the structure itself.

The god Heimdall is stationed at Bifrost's Asgard end specifically as its guardian, described in the Eddas as needing less sleep than a bird and able to see a hundred leagues by day or night and hear grass growing on the earth or wool growing on sheep, qualities that make him the ideal sentry for the one crossing point into the gods' realm. He carries the horn Gjallarhorn, kept ready to sound the alarm the instant any hostile approach nears the bridge, and this watch becomes decisive at Ragnarok, the prophesied end of the world, when Heimdall's horn call announces that the final battle has begun. The same prophecy holds that Bifrost itself does not survive the event: Muspell's sons, fire giants riding out under Surt's leadership, cross the bridge in the final assault, and the Eddic sources describe it breaking apart under them, unable to bear the weight and violence of that crossing even though it had held against every giant incursion before. The destruction of Bifrost is treated as one clear sign, among several, that the world order the bridge helped maintain, keeping the giants' realms separate from the gods' own, is ending along with everything else at Ragnarok.