There are two things that I really like in my games be they RPGs or otherwise: Deep sea, and high skies. I don't know what it is about them, but something in those settings fascinates me. I think it is part of why I love space opera a s well, since Space is basically the deep sea plus the high skies.
All three have one thing in common: it is very dangerous to go outside. In f act, going outside without proper safety gear is probably fatal. Either you'll drown, be crushed, fall to your death, or otherwise be killed by the environment. Which also makes them dangerous places for battles and RPGs.
Any time you put a high cliff, or a void, or something from which there is no easy coming back from in an RPG you have to be careful. Remember, the rule is that if something is in the game it will be interacted with. PCs will throw monsters off the ledge. PCs will fall off the ledge. Both of those tend to have the same result - the character going over the ledge is dead - but that's a much higher cost when it is a PC as opposed to random monster #6.
So how do you deal with it?
In my experience your options are limited. You just need to be careful. Even being careful you have to accept that it is something that can, and likely will happen - especially if you are counting on it not happening. You can't go for the drama of people fighting near a sheer cliff drop off, and not expect the dice to go "yeah, that's great...but what if they actually fell?"
So be aware of it. Be careful of it. And if, like me, you're drawn to these scenarios like a moth to flame then be especially careful and try to use it sparingly. Also, if you know you're doing it, build in some ways so that it's not just an instant death if you go over the edge or outside. A series of saves or skill challenges can make a difference. But if you put enough ledges, voids, or spheres of annihilation in a game...at some point, someone is going to end up thrown inside.