So, inspired by this post, I've decided to avoid my initial idea for a post -- to be written in in-character tone -- on the ancient history of Enigmundia told from different viewpoints.
Instead I'll just write out all my thoughts at once.
Those Who Were Cast Out are the devils and demons of the universe. As to where they were imprisoned it is a place separate from the Presence of the Creator.
Those Who Fell transitioned from being mere spiritual creatures to being partially of the material universe as part of their escape from the War in the Heavens. On Enigmundia, they began manipulating the natural laws with their celestial knowledge, taking on mortal women as wives, siring children (the earliest giants), carving out many kingdoms across the land, and teaching their increasingly mortal numbers the secrets they brought. Also, for a short time, they fooled or coerced the various Heralds and Warders of the world into recognizing their false authority. When the Creator and his loyal Choirs resolved matters with Those Who Were Cast Out, eventually caught wind of the anomalous reports from Enigmundia (and other realms) igniting the War in the Realms.
High Elves and High Dwarves were among the ranks of the Heralds and Warders of the world, and the War in the Realms caused many schisms and much confusion. As it continued, ancient secrets, strange wonders, and warped powers erupted in the service of these many factions. In the aftermath, the High Elves and High Dwarves disappeared from public view -- and the more familiar forms of the 'modern' elves and dwarves began appearing.
Let's see. And the halflings were mortals creatures brought into the world to kill abominations like the giants and chimerical creatures.