Questions asked about Arcydea and my answers to them, until the content is moved to a more appropriate section.
The short answer is that there are regional variations in temperature and terrain, but the world was forcibly reshaped into a spherical form during the Fourth Age because someone had the bright idea to blow up the moon of the goddess who had been holding the previous world's magic together. The 'sun' of the world is a light source, and is actually orbiting the planet; the world itself is heated in part as a byproduct of mana/magical activity. So regions that are more highly magical are prone to be more tropical, but the standard Earth assumption of tropical regions near the equator doesn't quite work in practice. However, on a mile-by-mile basis, the transitions between terrains generally make sense. There is technically an arctic region to the extreme north and south, though again, that's a magical byproduct more than anything – one section of the world known as the Nevergreen Mountain Range (technically still is, but the name doesn't fit the climate now) was a place where magic didn't work (because something in the area was sucking it all up, basically.) As such, it was a deathly cold frozen wasteland until the thing sucking up all the magic was found and destroyed.
There's another important thing to know about the world, and that's Nexus.
While the Black Rose Inn is essentially a small parcel of land contained within a pocket dimension, Nexus is basically a rip and fraying region in the dimensional fabric of the planet. That was created during the First Age – and trying to seal it only made things worse.
On the plus side, it's brought many interesting people from many different worlds, fantasy and otherwise, which is where the world originally got a fair amount of its technological trappings – though centuries and millenia of innovation and destruction have made technology development very elastic (you might meet any of the classic DnD types, but also a crossbow developer who makes man-portable ballistas, a ranger with a blaster rifle with a spyglass tied to it as a crude scope, for example, a gunslinger who handloads their flintlocks, a cowboy with six shooters, a guardsman with a rifle, and a programmer who communes with the ghosts in the machine – each from different regions of the world.
That's one reason I'm starting out with RAW characters – to give people an opportunity to see the weirdness as outside observers (and see what sections need better polishing; a fair chunk of this has been hashed out through the Ages – and using prior game systems.)
The original civilizations established on Arcydea were established by a group of deities collectively known as the Old Gods, who took an empty world that had already been created, and put their own ideas of how a world should be into practice, establishing creation of most of the original sentient races. At the time, this was a fairly insular demographic - the elves were uppity, the dwarves were isolationist, the humans were gregarious… you know the stereotypes. Then the nascent god of the Void found a small hole had opened in the fabric of the world. And, being as it was his responsibility, he tried to close it.
Unfortunately, the medical tactic of putting pressure on a wound to seal it does not hold a balloon with a leak closed, it just rips and stretches it open more. The hole became an actual leak through which unfortunate dimensional travelers with no destination - or random destination - started falling. Which was very unfortunate for the now completely unknown civilization that occupied that area, as it was destroyed and swallowed up, and the land that took its place was an ever-changing mass of city and countryside that eventually became known as Nexus City and the Backwaters.
It also indirectly caused the first Godwar and eventually the death of most of the Old Gods, but that's the Second Age and a story for another time.
But what becomes important to know is that these dimensional foundlings have had dramatic impacts on the landscape that were in no way planned or intended by the Old Gods, such that many of the trappings they established were destroyed. There are a few nations that can count their history all the way back to then, in some form or another, but there were many more adventurers who emerged from this territory to explore the world, and more looking for a place to take over and boss around.
The territory immediately around Nexus City is known as the Backwaters because for about six thousand years running, it has been constantly changing and rechanging hands as various micronations have formed, lived, fought, and died. There are a few nations that were once Backwaters - typically at the outer edges - that actually managed to take hold and permanently establish themselves, such as Avalon – established by someone from Earth who wanted to rebuild the glory of King Arthur, though now its royal family is elven as the lineage was broken early in the Third Age.
However, in recent history, there was a culture that was attempting to… well, basically, rule the world, known as the Traveler's Reach. Unfortunately, they were also worshipping an ancient evil god from the dawn of history while pretending to worship one of the major gods, and using that worship to gradually chisel away the other gods' religions to create a monolith of monotheism. And when I say evil, I mean it.
Over the past century, that power was broken, but it has still left deep scars on civilization, and there are still hidden enclaves that would like to restore their ancient evil god to power.