|Prime Material Plane|
Oerth was the original home of the lich Vecna, who now lives on the Quasielemental Plane of Ash, in a citadel bordering the Negative Energy Plane. This Prime world is of special interest to a number of powers, not in the least of whom are the drow goddess Lolth and Iuz, a demipower who originated here. The chant says that there once was a wizard here so powerful (and foolish) as to temporarily imprison some Abyssal Lords (including Graz'zt), a proxy or two, and even some powers' avatars. Not surprisingly, no one's heard from him in a while.
The world itself has a few permanent gates, but native heroes attempt to destroy or lock them when they can - the people here have a healthy fear of planar beings. A few that might still be open include gates to the main Elemental Planes, a conduit to the Abyss, and a few portals to the Lower Planes in a ruin called Castle Greyhawk.
Setting Resource: Greyhawk Wiki