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