A Korean fortress is like a super strong and big castle that people from Korea built a long time ago to protect themselves from enemies. Imagine playing with toy blocks and building a big tower that can keep you safe from bad guys. But instead of toy blocks, they used huge stones and walls that were really hard to break. The fortress was like a big circle with tall walls all around it. There were watchtowers built into the walls so the people inside the fortress could see if any enemies were coming. And there was usually only one way in or out of the fortress, through a big gate that could be locked up tight. So the people inside were safe and protected!