The short answer: the world Eric Grayson finds himself in, after being pulled through the Jarmuthe Chasm.
In a bit more detail – Eshla is divided into roughly four regions: The Jarmuthe region is mainly meadows, moors, hills and dales with some forest. A large proportion of the plants used to power Eshla come from here.
The Cedwigod region, is mainly forest with cities and houses built into the trees. A green, energetic light flows around these beautiful homes. Anna lives here in the lively city of Pren.
The Caregon region, is the hardest land – mainly rocks and stone but the mysterious Bel-Arayba cave lays at the heart of it.
Finally, the Elgion region, is made up of salt water and numerous water-islands with elegant canals.
At the center of all this is Hendref, the royal city, with its unusual skyline of distinctly shaped buildings, in a warm butter coloured stone, and flying machines which weave in between them. Hendref is covered in copper and brass pipes and mysterious humming machines. This is where the Arwain family have their home: Hendref Castle.