I kind of meant to have that done before I picked the players...
So the Western Empire is composed of the lands that used to be four kingdoms (Including the one that conquered the others), while the Eastern Empire holds the lands of three.
Both are constantly taking land from one another, and would love to conquer all the other remaining kingdoms, but for some important reasons have not been able to in 5 generations of war.
Sandiron was completely untouched by war until the desert started blowing away. They are naturally defended by the mountain ranges, the desert, and their neighbor to the west being the dangerous Black Hills.
The Black Hills is home mostly to dwarves and dangerous savagery; the dwarves are not savages themselves, but those that share the land with them are. They are protected from the west by mountains and everywhere else by being damned dangerous. They are content to stay put if you don't bug them.
Pigsley is a rural kingdom with mostly farmland; even their small desert is used agriculturally. They are defended from the Empires by the Empires; they're too busy fighting each other to conquer Pigsley. Also, there's their leaders, the Charging Boars, who personally guard the borders while making important decisions for the kingdom.
Skyfair is only accessible by navy; the Eastern Empire would have no problem with this but there's the issue of stopping to resupply, which has to be done at... Sandiron. The Western Empire is closer but is less advanced in naval war.
Bluewood is an elven wood. Obviously it's well defended by the natural magic of the forest.
And so these five kingdoms stand unconquered by two growing empires who would both love to paint more of the map in their own nation's flag.