Mountains can be very difficult to cross. They are often rugged and filled with forests and wild animals, such as bears and wolves. They may have no natural 'passes,' or easy places to cross the mountains. Mountains can also be hard to climb or may have ice or snow or glaciers that make travel dangerous.
Explanation:
All this means that crossing over mountains - to trade goods or to fight a war - can be tough to do. Sometimes, people who live surrounded by mountains feel very isolated from the world around them. It is just too difficult to cross over to other lands.
