The Bifrost

Before the birth of the World Tree and the creation of the Shadow Roads; travel between the realms was long and difficult. To circumvent this the Aesir created the Bifrost; a magical force that could be tapped into for dimensional travel. Much magic today that mortals use to travel between the realms actually is tapping into the power of the Bifrost and without it they would be unable to do so. The Bifrost Gate within Asgard is so powerful that when it opens it can be seen as a rainbow in the sky, allowing the Aesir to cross its prismatic bridge to their desired destination. Heimdallr oversees the Bifrost just as he oversees all things; it is through his will that allows those to travers the Bifrost Gate.