Asgard

Asgard is a world mentioned on Once Upon a Time. It is only mentioned in the fifth episode of the fourth season.

Asgard is based on the realm of the same in the Nine Worlds from the Old Norse religion. It is also based on the realm of the same name from Marvel's Thor and its subsequent films.

Background
Not much is known about Asgard, except the existence of the Mjolnir hammer which somehow ends up in Rumplestiltskin's possession in Misthaven.

Before First Curse
During his Dark One days, Rumplestiltskin pays a visit to Asgard. The Mjolnir hammer, which hails from Asgard, ends up in Rumplestiltskin's possession and is brought over to Misthaven. ("Skin Deep", "The Snow Queen")

Ways of Travelling

 * Magic beans can open portals to Asgard. (mentioned in "Breaking Glass")