Ever-vigilant Heimdall, an Aesir god with keen eyesight, guarded the entrances to the bridge, and stood ready to sound the Gjallarhorn should enemies slip by. According to the prophecies of Ragnarok, Heimdall would blow the Gjallarhorn when the armies of the jotunn invaded Asgard, and the Bifrost would crumble in the ensuing battle. More @Wikipedia
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