A wall is a structure that defines an area, carries a load, or provides shelter or security. There are many kinds of walls such as defensive walls in fortification, walls of buildings which are a fundamental part of the superstructure or which separate the spaces in buildings sections sometimes for the purpose of fire safety, walls which hold back earth called retaining walls, offer protection from oceans such as a seawall or river as a levee. Permanent, solid fences are walls, and border barriers between countries are sometimes walls.