Arth was my fantasy world when I started GMing The Fantasy Trip (a viable alternative to ADnD... would be known to you as GURPs v0.0) back in 1980. I ran Arth for groups of various sizes (1-20) at least once a week for 8 years. It was hugely popular game due mostly to my GMing abilities (Storytelling and Character generated storylines back in the day when you had to convince people to have a reason and theme for their dungeon rather than just a clever set of traps and monsters). Why am I babbling about this? I found some of my old notes. They would mean little to most people, but they triggered the encyclepedic work I did on the world. Since this was "back in the day", I was winging everything with scant notes and feeble game support. I had yet to discover cue cards, world packs, super sheets, dice heraldy or any number of things to make a game run easier. (That all started in 83 when I was doing convention work for Tri-Tac and Fringeworthy and Hero System). This world was a great one. I should bring it to life and provide it for others. So hopefully you think it is as interesting as I do.
The World is completely outlined (in very broad strokes) in the Arth main thread. The smaller subsections will have details and more complete write ups.
The Beauty of this world
The beauty of Arth is that once you have the basics, you can run your game outside the KnownWorld. Arth gives you some "world mechanics" that allows you to have different areas with different rules in them. This gives you the maximum freedom in your design considerations. If you want a high tech area, but don't want phasers messing up your fantasy world, you can have it, by defining StarMagic/ Technology past a certain point only works there.
Of course, the KnownWorld may be that mythical place... overthere somewhere for your own world. You can even have your world be "imported" to Arth, without a problem.
Inspirations for Arth... Well of Souls series (though Midnight was the only one written at the time), Land of the Lost, and the fact that I had more ideas than could be contained in a conventional world. Originally I thought I would just be copying things, but I found out my own ideas were more than enough.
As I have been reconstituting this world, I realized I am actually refining it. Given decades of gaming experience, I am making minor changes, adding things to make the world better, and filling in the gaps where vague references were before.