Okay, I'm just going to hope my players don't think to read this.
Please help me by commenting. What makes sense, what doesn't make sense, and more importantly How much destruction can I imagine would have happened in the last 20 years?
For the layout of the old kingdom consider france. There are 22 large cities each controlled by a high ranking knight. In each "provnice" there is exactly 1 large city, so 22 large cities, 22 provinces. The area is walled in on the east and west by impassible mountains. To the north it degrades into tundra just north of the northern provinces. To the south it degrades into desert making this kingdom the entire known world. At the northwest corner there is a place where the mountains end where the party will be "coming from". To the east, there is a pass through the mountains that is unexplored. The entire kingdom is forests and plains.
The problems started 25 years ago when the mines in the southeast corner ran dry. Within' five years there was no new mines from which to get iron ore. This left a fuedal kingdom starving for iron. At this humanoid invaders began to spill in from the pass to the east through the mountains. The province that "guarded" the pass fell quickly allowing the humanoid army to strike at the surrounding provinces. The old king died during this time, replaced by his not as battle hardened son. There was an increase in scheming and infighting. The citizens of the three mining provinces to the southeast began to move outwards and upwards (since the mines were gone, there were no jobs. With no money from ore export, they could not import goods). This caused TREMEDOUS overcrowding in the surrounding provinces. While this was happening a plague sprang up in the southwestern provinces. Citizens who learned of the plague began to flee to the surrounding provinces (the ones already being overcrowded with job seekers from the southeast). Some of these fleeing citizens brought the plague with them. At this same time in the northwest, a strange warparty of questionable origen sacked the cities in the northwest. These were the lumber provinces, so the supply of lumber was cut off. Without ore and lumber, the central kingdoms had little left over to fight with, and almost no way to prevent the plague from spreading that way. Already overburdened with civil unrest, a faction sprang up to try and create a coup d'etat. The capital Province was caught between the rebels and the invading army. Eventually it collapsed, leaving the rebels and the humanoids to destroy the middle country with war while the plague reduced the number on both sides. Eventually the humanoids would back off to the original provinces they conquered, leaving the wartorn, plagued central area to waste away to almost nothing.
Now, one important thing to keep in mind is that this is a very low magic world. Wizards of more than level 10 are almost never seen.
There is no contact with the outer planes, so summoning spells, many transport spells, etc. do not work. Also, most clerics are limited to spells of level 3 or lower. Even higher level clerics are banned from raise dead and ressurect.
Finally, Dwarves are fully and completely integrated into human society (there are many half dwarves). Elves are in another place where the pc's come from and do not have any impact on this kingdom. Finally, gnomes and halflings do not exist. The only two races in this kingdom are Humans and Dwarves (and their crossbreeds).
So with that in mind, what would the situation after 20 years be?