Can someone judge my homebrew setting?

I've got this setting I've been thinking about for some time, and I was wondering if I could get some feedback on what you guys think about it.

The grand continent of Arrokos was once a land of unity, before the great war. The great war divided the land of Arrokos between its various peoples, the voices of those who advocated for peace drowned out by the shouting of warriors and the clash of steel. In the end, no one party won the great war, and no scholar can agree on what caused it in the first place. All they can agree on is what became of Arrokos in the aftermath.

To the northeast, the elves created their final settlements hidden in the Ageless Forest, building a mighty castle among the trees and declaring themselves the masters of the continent, believing that they had proven themselves as such during the war. Their soldiers patrol the streets of other settlements, attempting to wrest control from the other nations. To the elf king, all of Arrokos is his domain, and those who do not follow his command are but rebels, pretending at control and nationality.

To the northwest, the goblins hid within the twisting mazelike caves underneath the Black Slate Mountains. Before they disappeared, their king vowed to the rest of the continent that he and his legions would return with machines of horrific destruction, and that they would not be defeated ever again. No one has seen a goblin since, and any who try to enter their domain are stopped by the unclimbable cliffs that surround their domain, the lone way through barred by an incredible wall of black steel.

To the east settled the hearty dwarves. Nestled in the seat of the Goldtail Peaks, the dwarves mine and build, harboring a deep hatred in their hearts for the elves and the goblins, who they know would love nothing more than to rob them of the resources in their mines. They guard their riches jealously, and rob any who dare approach them.

To the south live the Dragonborn, who chose to withdraw from the rest of the continent at the end of the war. Their mighty mages used ancient magic to split the Rainbow Archipelago from the main landmass, turning theirs into an island nation, protected by a host of vicious monsters that prowl the seas around Rainbow Island.

In the center of the continent, live the human race. Their foolish king keeps the trust between the other nations thin, fearing that if they are not at each other’s throats, they will be at his. They control the only convenient way over the great river that separates the land in two, a mighty wooden bridge that spans the river’s prodigious width. The humans permit passage across their bridge, for a price.

After the great war, the great many that had perished were reanimated and assembled into a new sixth nation, the undead. A being known simply as “The Apparition” leads the horde and restrains them from unleashing their hunger on the rest of the continent, claiming the ruined elven towns and castles to the southwest as his kingdom, renamed “The Nether City.”

Rumored widely among all six lands is a legend of ultimate knowledge beneath the earth of Arrokos. Tales are whispered in hushed tones of a host of demons living far beneath the surface, beings that have watched the lands above since time immemorial. Those who still hope for peace speculate that the all knowing fiends hold the answer to the mystery of what caused the great war, but who would be brave enough to risk facing such monsters to find out?

I will say picking your race/species will be a big factor in such a campaign. Normally such decisions are more about strategy and flavor, but here picking one could really determine alliances and enemies before the adventure even begins. Plus it could cause problems if two opposing races/species are in the same party. It’s not a deal-breaker by any means. It’s just something important for everyone to keep in mind when deciding their characters or when you write out the adventure in more detail.

Hope that helps!

Looks really good and well thought out

Sounds like you have identified your factions, and sounds like you’ve set up your mystery/unknown about what caused the war. And you’ve set up the demons as the holders of the answer to the mystery. How will the PCs learn of the demon hook? What are the PC motivations for discovering the cause of the war? Besides the demons, are there antagonists? If there is a faction that has an interest in the PCs not discovering the cause of the war, that could add a layer of complexity to the campaign.

Re the extent to which the race species (or race) of the PCs will affect the PC dynamic, this is a DM decision. If the DM is stipulating rigid and traditional conceptions of playable species for the campaign, and there are PCs with different species, then it could be a potential problem. D&D has moved away from rigid and traditional conceptions of playable species.

I agree with actionsparda about the challenge of having all these species split up with pretty firm lines between them. Are you going to require the whole party to be of the same species? I know it’s pretty common that PCs are exceptional, and even if elves and dwarves don’t get along generally, within the party you can easily have an elf and dwarf who are best friends. But how are they going to all meet up?

Or maybe I’m misunderstanding and traveling and intermingling is more common.

This sounds something like the Last War on Eberron, where the nations are all split after a century of war an each are jockeying for advantages when conflict breaks out again, while fiendish and aberrant forces work behind the scenes to bring power and benefit to themselves. You may want to check that out for inspiration.

Also, are players allowed to play as undead, or are they restricted to the other five. And for that matter, are there only the six kinds of creature and players are limited in character creation, or are there other, far less prominent species trying to carve their own domains.

Consider the abundance of magic. Are mages common everywhere, or only with the Dragonborn. Which countries shun what kind of magic. Prominence of magic items. Magical creatures. Are there lots of dragons, or are they extinct. 

Are games played in this setting darker, sword and sorcery slaying demons, or political, where you don't know who to trust. What is the mood and tone of the world, and what genre does it lean into the most.

Just some basic questions about specific features of your world that are not mentioned in your overview.

It's a fine framework for a setting, but as the others have mentioned it's not super clear what a campaign in this setting would be about (that's not necessarily a bad thing as a good setting should enable multiple different campaigns to be played in it) and especially how a party of disparate species would be able to work together. 

For both I would suggest thinking about how the world looks to the commonfolk, not just how it looks to the kings of each nation. So while the king & authorities of the dwarves might fiercely guard their nations resources and fear the other species, that isn't necessarily how the a dwarven commoner would see the world. [Just look at politics today to see how rarely the priorities the political leaders align with the priorities of the people]. 

The level of magic, technology, and wealth in each nation is also very important to establish. Are all nations equally powerful but with unique approaches? Or are certain nations more/less powerful or rich than others? Are there mages walking around every mid-sized settlement or are they rare elites? Or is one nation more magically inclined than another? Are there magic shops on every corner, or are magic items rare relics of high importance to the authorities? 

Lastly, you should consider how the various classes interact with the world. D&D has god-related classes in the Cleric and Paladin, so what gods are present in your world and how are they worshiped? Are religions integrated into the politics of each nation with certain nations worshipping different deities and religious leaders closely tied to political ones, or are they kept separate with religions spanning across multiple nations with little relation to politics? Similarly Druids are supposed to be linked to Druid Circles, and Monks are supposed to be linked to monasteries, are they so in your world? If so where are those circles and monasteries and are they linked to specific nations or not? Other class-related organizations you might want to include are thieves / assassin guilds (in a cold war situation spies & assassin are usually very powerful), Bard and/or mage colleges and schools, militaries or mercenary groups for fighters/barbarians/rangers, crafting guilds for artificers.