Envision a spiralling staircase, lacking beginning or end, with doors on all sides, and know that this is the root of all things, the Stair of Worlds. Know also that the gods arrived from elsewhere, and before their coming this place was part of the place beyond, part of the Far Realm. Know that the seeds of the Far Realm were sown through the Stair of Worlds, which connects all realities, and know that one of those realities retaliated. Know this and understand the Godswar dawn of the first age.
The gods were initially victorious, but rapidly realized that they would lose the war to attrition. Instead, they built a great fortress around the Stair of Worlds, and protected their fortress with an intricate set of wards. In doing so they created Cyteria, and its attendant planes. This puzzled the gods, and many of their number retired to consider this development. Others played with their new realm for a while, and several ages went by, when one empire or another would rise to prominence. One by one the gods lost interest with their creation and returned from whence they came, until only Pelor remained.