A fort is a place that's made strong and secure enough to be defended during a war. Although it’s usually a military word, you can also make a fort with chairs and a blanket to defend against annoying little siblings.
A fort can also be called a fortress or a fortification, although typically a fortress describes a larger structure or a series of structures, while a fort is usually only one. All of these words come from the Latin word fortis for "strong, mighty, or brave.” Traditionally, forts have been strongly defended, with thick stone walls and other elements protecting them from invaders. Today in the US, some forts are simply military bases. Others are simply in the living room.