Hello, everyone!

My first attempt to swagger into the giant primary forum, and I come baring a question I hope hasn't been asked too much. For the sake of discussion, here's a scenario.

There's a fantasy world, as there so often is of course. This world, however, is not a planet. In fact, it is less of a world and more of a realm of existence. It is grounded firmly, however, by the laws of physics. Because if it wasn't, how could we even do anything in it, really? The space not arbitrarily finite or infinite, rather it is open to expansion or even extinction based upon the whims of those whom sadly control the fate of this world. Godheads, as it were.

Let's add that it happens to be a bit Tolkien-esque in the fact that the oceans, or ocean, could lead to an entirely different realm of existence. So in essence we have a world that is flat. Not paper flat, of course. There has to be topography. Mountains, hills, forests, rivers, lakes. How would you approach such a creature? Surely it would be easy to just make a regular, old-timey looking parchment map and ignore the overarching lore, but what massive factors missing from a non-rounded, non-planetoid surface could influence the way topography works realistically? Things that might not be immediately obvious to laymen but that should be considered in terms of having any sense of grounded realism to the terrain.