A tower is a tall structure, taller than it is wide, often by a significant margin. Towers are distinguished from masts by their lack of guy-wires and are therefore, along with tall buildings, self-supporting structures.Towers are specifically distinguished from 'buildings' in that they are not built to be habitable but to serve other functions. The principal function is the use of their height to enable various functions to be achieved including: visibility of other features attached to the tower such clock towers; as part of a larger structure or device to increase the visibility of the surroundings as in a fortified building such as a castle; or as a structural feature as an integral part of a bridge.Towers can be stand alone structures or be supported by adjacent buildings or can be a feature on top of a large structure or building.