The Avengers: Earth's Mightiest Heroes Wiki
Heimdall - Appearance

Heimdall guarding the Bifrost Bridge

The Bifrost Bridge also known as the "Rainbow Bridge" is the main gateway of Asgard to all of the Nine Realms. The guardian of the Asgardian bridge is Heimdall. It was used by the Asgardians to travel in and out of Asgard and to visit other realms they have never seen before.

It is believed that the Bifrost can be accessed by Asgardians but, however, the mortals from Midgard found a way to access this mythical bridge like the Red Skull who found a way to use the bridge to pull monsters and other creatures from the realms in his plan to unleash them against the Allies but was stopped by Captain America and Bucky in Norway during World War II and Iron Man who designed a portal machine to Asgard but it was never tested.

The Norm Stones can be used as an alternate route to other realms if the Bifrost Bridge is unavailable.