Imagine building a big castle made out of very hard and heavy rocks. This castle would have walls and towers that are really strong and can't be easily knocked down. It is like a giant puzzle where all the rocks are put together carefully to make the castle. Some of these castles were built a very long time ago, and people would live inside them to stay safe. Today, some people still build rock castles as a way to remember history or to make a really cool place to visit.