Although most cities seem to form by accident, for thousands of years some of them have been designed. Whether for defence, beauty, or practicality, urban designers have imposed their ideas of what a city should be about. But ideas are subject to changing needs and fashions. Centuries ago, a moat or a castellated wall would have been essential. Now, greenery is in vogue. While existing cities look for ways of becoming more environmentally friendly, a number of new ones are planned that intend to be totally green. One of these is Masdar. Masdar's advertising states that "one day, all cities will be built like this." This is not the case. For one thing, Masdar is experimental and a work in progress. What emerges will not necessarily translate well elsewhere. Each green city is unique, and getting it to work depends on its location and economy.