Cosmology
At the intersection of the four elemental planes lies the Physical World that we all see and know. Inter-laced with that Physical World is the Spirit World, inhabited by the spirits of the departed, the various neutral gods, and a host of other lesser spirits. While it is relatively easy to summon creatures from the Elemental Planes using spells and magic items, travel to those planes is only accomplished by entering fixed portals on the earth where that element is strong (e.g. there might be a portal to the Plane of Water at the lowest point of a sea or deep lake).
Travel to and from the Spirit World is more common. Spiritual Undead exist in both planes at once. Light Elves can travel to and from the Spirit World at the hours of Sunset and Sunrise. On Halloween, after sunset, various magic spells can accidentally open portals to the Spirit Realm, and various Spirits have easy access to the Physical World. The gods who live in the Spirit Realm can pass freely between the two worlds at will, and can cause others in their presence to do likewise. Finally, there do exist many ancient magic portals that permit the crossing into the Spirit World at will.
Above these 6 planes lies a single plane of pure Mind, known as the Transcendent Plane, or The Heavens, which is inhabited by the single Transcendent god, known in the common speech as Bor. He is surrounded by his angels, archangels and the souls of the saved. The Transcendent Plane is reachable by travel through the Astral (or Starry) Path, which is incredibly easy with Bor's permission, and impossible without it.
Beneath the Physical Planes are an infinite number of maddening planes, known collectively as the Abyss. The Abyss is inhabited by an infinite variety of demons, devils and hellish monsters. The Abyss is reached by travel through the Black Path, which is incredibly difficult and requires ritual and sorcery to accomplish, and this fact alone keeps the Physical World from being swamped by hordes of demons. While the bodies of demons rarely enter the world, their spirits are more easily called. Many of the monstrous creatures of the earth (Orcs, Trolls, Ogres, Goblins, even some Undead) are actually animated by minor demonic spirits called forth originally by sorcery.