A fortress is a large building or complex of buildings used as a military stronghold. In a military sense a fortress is often called a “fort”.
From its original sense of stronghold, the word fortress has stretched to include strongholds in a more figurative sense. Martin Luther used fortress to describe unfailing spiritual support when he wrote, “A mighty fortress is our God.” When someone has a hard time trusting others, you might say they have retreated to a "fortress of their own making."
