Asgard has always been attacked by evil forces from the realms intending to take over while Odin went into the Odin Sleep to replenishes his Odin Force powers but were stopped by Thor.
The Asgardian kingdom was conquered by Loki and his allies while Thor remains on Midgard (Earth) because of Loki's releasing captured supervillains from their prisons just to distract him.
The Asgardians who were captured or remained hidden rebelled against Loki's rule thanks to the unlikely arrival of the band mortal heroes from Midgard known as the Avengers. Together, they defeated Loki and trust was achieved between mortal heroes and Asgardians.
Asgard was somehow visited by Beta Ray Bill by accident as Odin summoned the wielder of Mjolnir.
Realm Below a subterranean cavern complex beneath Asgard.Ulik traveled through a portal from his realm to Midgard, with the Codgel in his possession. The portal was later reopened by Thor, to trap the Midgard Serpent and Doctor Doom to the molten pits of the Realm Below