A fortress is a large, heavily defended building or group of buildings designed to protect people from attack. Unlike a simple wall or fence, a fortress is built with thick stone walls, strong gates, high towers, and strategic positions that make it extremely difficult for enemies to break in. Medieval castles were fortresses, as were the massive stone forts that protected important cities and harbors throughout history.

Think of a fortress as the ultimate defensive structure. While your house has locks on the doors, a fortress might have walls twenty feet thick, a moat filled with water, towers where archers could shoot arrows, and only one narrow entrance that defenders could easily protect. Some fortresses were built on clifftops or islands, adding natural defenses to their human-made ones.

The word also describes anything that seems impossible to break into or change. A person with very strong opinions might have a fortress of beliefs that no argument can penetrate. A company that dominates its industry so completely that no competitor can challenge it has built a fortress in the marketplace. When basketball teams play fortress defense, they protect their basket so well that opponents struggle to score.

Throughout history, fortresses changed the outcome of wars. A small group of defenders inside a well-built fortress could hold off a much larger attacking army for months or even years.