A fortress is a big, strong building that people use to protect themselves from danger. It's like a giant castle with really thick walls and lots of places to hide inside.
You know how your parents tell you to lock the doors and windows at night to keep bad guys out? A fortress does the same thing, but on a much bigger scale. It's made to keep enemies out and protect the people inside.
Fortresses were built a long time ago, when people didn't have all the fancy weapons and technology we have today. So they had to come up with lots of ways to defend themselves without guns or bombs. They built moats filled with water to make it harder for invaders to get in. They put big gates and walls around the fortress so no one could just walk in.
Inside the fortress, there were often rooms where people would store food and water in case they were under attack for a long time. There might have been a big tower where someone could keep watch and see if anyone was coming.
Overall, a fortress is kind of like the ultimate safety zone. It's a place where people can go to be protected from harm, where they can sleep well at night because they know they're safe.