A wall is a structure that defines an area, carries a load, or provides shelter or security. There are many kinds of walls: * Defensive walls in fortification * Walls in buildings that form a fundamental part of the superstructure or separate interior sections, sometimes for fire safety * Retaining walls, which hold back earth, stone, or water * Walls that protect from oceans (seawalls or rivers levees) * Permanent, solid fences * Border barriers between countries * Brick wall * Stone wall * Glass wall (only when most of the wall, in smaller amounts it is called a window) * Doors are mobile walls on hinges which open to form a gateway