An alpine fortress is like a really big castle that is built on top of a really tall mountain. It is made to protect people and their things from enemies who want to take them away. It is built in a place that is hard to get to, so that enemies have a hard time trying to attack it.
The fortress is usually made of strong materials like stone or metal, and it has thick walls that are hard to break. It also has a lot of towers and gates, which are places where guards can keep watch and make sure nobody sneaks in.
Inside the fortress, there are usually all kinds of things that people need to live, like houses, water sources, and storage rooms for food and other supplies. There might also be things like weapons and armor to help protect the people inside if they need to fight off an attack.
Overall, an alpine fortress is a big, strong building that can protect people and their things from bad guys who want to cause trouble.