Ksar is like a big sandcastle that people used to live in a very long time ago in a special place called the Sahara Desert. It's made out of clay or mud and sand that's been packed together really tightly. That makes it very strong!
Inside the ksar, there are lots of small rooms and winding paths that make it like a big maze. People used to live there because it was cool inside, even though it was really hot outside in the desert. They could also store their food and water there safely.
Nowadays, people don't live in ksars anymore, but they are still really cool to see and explore. Some of them are very old and have been around for hundreds of years! They're an important part of history and culture in the Sahara Desert.