There is no clear, simple, universally accepted definition of ‘city’; in some developing countries urban areas are distinctly different from those in richer nations, and within each country cities differ, sometimes markedly. There is usually considerable difference between districts of any city. Some cities have been planned, and in some cases built on undeveloped rural sites. Many cities grew up in a wholly haphazard way, and reflect economic, defensive, or colonial needs. There are cities established for centuries, and some built within the past few decades.