A fort usually means a structure with walls and other defensive measures made to withstand an attack by an enemy. A fortress can be the same thing but usually means a larger area like a city built into a fortress with a wall surrounding the whole city. I built a fort but we made our city a fortress. Fortress would also be more commonly used to describe something you are building very stout. "You are building your house like a fortress" They are both nouns and have very very similar meanings, so the difference is very minor if anything at all.