The information below was simply gathered in local taverns from knowledgable commoners. They are just different perspectives on the place called Ysgard.
From a visitor from Sigil: Called Gladsheim by primes who never found out the real chant. Ysgard is a tumultuous plane. It's made up of numerous immense rivers of earth flowing forever through the sky. These rivers consist of chunks ranging in size from small boulders to whole continents. Their undersides give off a reddish glow that lights the rivers from underneath. As each river flows, its "earthbergs" grind together fiercely, so the whole plane continually rumbles with a deep grating noise. The plane's occupants are just as tumultuous. The Norse pantheon (sometimes called the aesir) dwells here, and it's a rowdy lot. Some think it strange that such chaotic gods would ever band together, but a strong leader and a common goal to maintain a home realm can be nearly as unifying as any universal law or code. Odin is that high-up man (though Thor is nearly as mighty and Loki nearly as crafty), and by standing together these gods preserve their realm within Ysgard's physical chaos. Petitioners on this plane are always eager to do battle. If they die in combat, they aren't absorbed like petitioners on the other planes; instead, they rise the next day to fight again. Unfortunately, they forget that other sods who aren't petitioners might be averse to a death duel. What's worse, most other bashers here are members of the Fated faction, and they figure that if a sod can't defend himself, he deserves to die. The moral of the story: Ysgard's no place to take a vacation unless a cutter's looking for a real workout.
Ysgard: Ysgard's also the name of the highest level of the plane, and it's the home of the Norse powers. This realm is where the top of the Yggdrasil - the World Ash - is located. As Yggdrasil stretches down to the Gray Waste, it extends branches and roots into various crystal spheres on the Prime Material where the Norse powers are worshiped. These gods also command Bifrost, a rainbow bridge that they can connect to any prime-material location. Two other significant features of this realm are Gladsheim, Odin's feast hall, and Valhalla, Odin's hall of heroes. Found in the layer of Ysgard are the realms of Vanaheim (home of the vanir, who have blood links to the aesir), Alfheim (where the most chaotic elven spirits dwell), and Jotunheim (a land populated primarily by giants).
Muspelheim: This layer of Ysgard adds another dangerous twist to the tumultuous nature of the plane. Here, the chunks of the earth float flaming side up. This makes it a perfect home for fire giants, but a miserable place for pretty much everyone else. Portals here lead primarily to Jotunheim.
Nidavellir: Nidavellir, known to the locals as Darkhome, is so densely packed with earthen rivers that the spaces between seem more like immense, luminous caverns. Sometimes this makes travel from one realm to another extremely difficult, as passageways that were open on a previous trip are closed on the return. Many dwarf and gnome petitioners hang their hats in this layer.
From a local bard, reciting the recent known history of the bergs(he was quite drunk at the time, so take this information with a dash of wyrmsage): A group of Ysgardians got fed up with Kord's realm. They built the first airship and flew out of his concentrated realm. They landed on an island known as Aulus Isle and built the first city - Nagritch - around a large crystal which seemed to give the powerless paladins and clerics back some of their ability. Kord decided to let them go, realizing that they'd probably just have a miserable time and come home eventually.
Several sects of the fleeing group were not happy with the way Nagritch was being run, and all split off. Each of these groups founded the other three cities of Hala: Nantes, Felmoran, Hangate. Each of these four cities refused to worship any sort of god. Nantes was named after the leader of the city, and he approached the status of Demigod. The mages of Nagritch obsessed with the crystal and built a tower around it so they could study it. Many of the mages fell into corruption.
After many years, Kord officially gave up on the Halatians and refused to exert any of his power to do anything for the island. At this point, the demons saw an opportunity to get a foot into Ysgard. The opportunity was taken, and they caused the 'Great Split'. In the days of the four cities, Hala was a vast green island with massive lakes, rolling hills, and no berg bridges attaching broken parts.
The Great Split changed all of that. It tore the land into pieces and scattered all the cities. Nantes was destroyed, Felmoran was destroyed, Hangate was cast into rubble and became more of a mining village, and Nagritch was the only thing that stayed up. Shortly after the split, Celestials started appearing more often around Hala and assisted the people with their rebuilding efforts.
Minya was cast into Hala by an unlikely happening. The Prime plane it was located on was destroyed, but the island itself was saved by Almara and was allowed to plummet into the inland sea north of Aulus Isle. Almara saved the island because He lived there for a long time when he was a child. Almara used Minya's unusual characteristic -- The Tree of Naught -- to rescue the people. He sucked the entire island into it, then gated the tree, and reversed the process. Naturally the impact of the new island flooded a substantial portion of Nagritch for a while. The most important aspect of Minya is that the people are devoid and frightened of magic. Their relatively new existence in Hala has turned their world upside down and for the most part, the people are still quite confused.
Mirtho Rest was built upon with the destruction of New Heavens. New Heavens was an unsteady island and as a result, buildings were moved to a more suitable location.
The Celestials and Demons wrapped up their Blood War(on this plane, at least).
Nagritch has had an uprising of chaotic good people, and ALL of the high LE officials of the city have been killed by several well-known adventurers. There are still a couple evil politicians and guard captains pulling the strings, so it's said.
MORE INFO TO FOLLOW