In legend it is said that Odin, using the help of the Valkyries, would bring those warriors who demonstrated bravery and courage, and came close to death during battle, to come to Valhalla. To one day join the rest of the Norse Gods in the final battle, known commonly as Ragnarok (the battle, not the clone of Thor). Valhalla is/was the opposite of Hel; where souls of warriors deemed not brave and courageous enough to join Valhalla would end up. And eventually fight in the final battle.
In the year 2099, there is an actual floating city dubbed Valhalla that brings about the Fall of the Hammer. Created by Avatarr, its main purpose was to house a group also created by Avatarr, known as the Aesir. Soon after, a battle between the X-Men and Doom with the Aesir took place, killing Heimdell in the process with the help of Loki. Afterwards Doom faced off against Thor, in which during the battle a huge explosion ripped the two apart. Causing the two to regroup, Doom joined a group which included 2099s Spider-man, Ravage and Punisher while Thor was reminded by his master, Avatarr, of his mortality. Doom's group soon took the city under control, until the city's engines maintaining it afloat failed and collapsed the city onto New York. During this time, the X-Men gathered the workers from the floating city and brought them to safety. After this the team of heroes from the 30th Century threw Thor's hammer into Valhalla's engines causing the engines to stabilize once more and keep the city afloat once more. In the end Doom claimed Valhalla for himself as a province of Latveria.