A tower is a tall structure, taller than it is wide, often by a significant factor. The cited article says towers are distinguished from masts by their lack of guy-wires and are therefore, along with tall buildings, self-supporting structures, and that towers are specifically distinguished from buildings in that they are built not to be habitable but to serve other functions using their height. For example, the height of a clock tower improves the visibility of the clock, and the height of a tower in a fortified building such as a castle increases the visibility of the surroundings for defensive purposes. Buildings recorded in the atlas as towers are grouped under this concept.