Spoiler, near the famous city the castle of Greyhawk is now a famous planar hub, and now the city of Greyhawk is like Sigil or the Radiant Citadel, receiven planar trader and visitors from other wildspaces.
Second spoiler: Within the city there is a secret demiplane working like a "mirror universe" with planar gates toward the "twin planets": Yarth, Aerth, Uerth, Earth.. These worlds also suffered their own version of the "Sundering" (and you could blame Vecna and the troubles caused by him in Sigil).