Some mountain ranges are only 3-4000 (914 meters) feet tall while others tower above 10,000 (3000 meters). This is a critical difference. Lower mountains are less likely to cause a rain shadow, while taller ones absolutely will; the surrounding terrain is affected for as much as a thousand miles. Travel across a range differs according to how high one must climb to traverse them. Survival conditions deteriorate, too, as some mountains are so high that oxygen deprivation occurs (above 2400 meters or 8000 feet). The increased cold is another factor; it is possible to freeze to death. An underground species like dwarves may tunnel through, as could monsters, to solve this problem.
The ability of flying species, giant birds or dragons, to cross these peaks might also be impacted, particularly if they’re carrying a load, such as our characters. We can narrate that our giant birds can take them as far as those peaks but no farther. The air becomes thinner at higher elevations and even birds struggle to fly over. The additional lift required by something the size of a dragon may preclude it from flying over the highest peaks, except that we like the idea that they’re all powerful, but the point is worth making.
The higher the peaks, the less likely they are traveled, meaning a settlement there might be safer than one at lower altitudes. This will determine how many visitors the place is likely to get and even how receptive the residents are to that. This can work in different ways.
For example, a settlement in high mountains might be rarely visited and therefore suspicious and hostile toward outsiders. Then again, maybe it has no reason to be so hostile because no one poses a threat to it. Perhaps a settlement in smaller mountains is the one that’s very protective and suspicious, refusing admittance to others. We can create different scenarios for each range, making the settlements therein different. Higher altitudes also pose problems with oxygen deprivation and higher mother and infant mortality rates.
Which species live in each mountain range? It’s likely that the same ones live in every one, but that may depend upon their attitude. I have a species that likes to prey upon other species but which is simultaneously at risk of being preyed upon by others. The result is that, if their prey isn’t present, they tend to not be there either. Some of our species might have massive numbers in the peaks, which they use as a base to amass armies or from which to launch raids.
Another issue is whether we have a species, or more than one, known for tunneling into mountains, like dwarves. They can have underground civilizations that mine for gems and minerals like gold. This can make their lairs desirable for conquest. They may also unearth monstrosities buried deep in the earth and better left undisturbed. Maybe their place has been destroyed as a result, with all manner of ghosts haunting the ruin. Or perhaps they’ve controlled whatever they’ve unleashed and can send it after those who try to conquer them.
Dangerous animals and monsters can also be present and impact travel. Their presence can be known or not. Do our characters prepare for an encounter or try to evade them? Do they assume a certain number of them will be killed before they get through? If the mountains are low enough and our characters can afford aerial flight, maybe it becomes a moot point, unless these creatures can also fly or knock out of the sky anything that does. Then our people must escape alive.
Below are two example descriptions similar to those you’ll write for your files.
These intimidating mountains are shorter than most, being less than 5,000 feet tall, but the stories of evil wizard towers and all manner of monsters have kept most people from entering. Those who do often don’t return or return insane, babbling nonsense about hundred-foot-tall cave trolls, talking trees, and rocks that come alive and chase people. The wizards of Nivera have investigated these rumors and found enough evidence to prove the claims, though they have admitted that only to the royal court in Nivera.
Volcanic in nature, these peaks still rumble with activity so that those nearby have long thought the gods must visit here. The tallest peaks are over 10,000 feet while the average is a couple of thousand less. Heavily forested, they are home to all manner of dragons and other flying creatures, many of them preyed upon by the dragons. The southern pass is inhabited by ogres and goblins, making travel hazardous and the city of Nivera hard to reach or escape. By contrast, the northern pass is well protected, courtesy of castles at either end and a legion of knights.