My home setting is the modified Flanaess, following the Greyhawk Wars, Solistari War, Coming of Vecna, and Departure of Vecna. It's been 100 years since the Departure of Vecna.
No normal farming is conducted anymore. Normal trade has been supplanted by magical trade (across heavily fortified borders.) Food is grown in magical retreats, or in secret places.
Those nations that remain, have large armies, magical or psionic might, and heavily defended frontiers.
Cities are now fortresses. Villages are within castle walls. The countryside around is abandoned, returned to wilderness (or cleared and leveled, and massively patrolled.)
The world is shrouded in smoke and volcanic fog. Some areas are freakishly hot, others freakishly cold. The weather of the world is overthrown, tossed aside in favor of magical modification and the effects of inland seas of lava.
Seas have uplifted into land. Land has collapsed into sea. Mountains have shattered and fallen, while other mountains have exploded upward. None of the old maps are of much use ... when you leave the safety of civilization, you're on your own in the unknown wilderness.
Since that wilderness is filled with danger and horror, most frontiersmen never return. Adventurers had best prepare themselves for the worst, if they wish to venture out.
There are isolated, powerful, and very altruistic (even exalted) enclaves of good amidst the general evil of the land. These act as sanctuaries for those who can reach them ... or as targets for the evil overlords of many nations.
Greyhawk City stands, amidst the altered Flanaess, but even one hundred years after the Depature of Vecna, it still bears his taint. It is a magnificent, wondrous, and very Menibonean city, wealthy beyond measure, imperialistic as the Great Kingdom once was with swarming armies and fleets of ships and spelljammers, and ultimately deadly.
Mordenkainen and the Circle of Eight still operate. Their base is Mordenkainen's Steel Citadel in the Yatils, now as fortified and deadly as the Tomb of Horrors. Mordenkainen and his fellows are very high level, very dedicated, and very ruthless in their efforts to return the Flanaess to some semblance of normalcy. Interrupt or otherwise bother them at your peril.