A hill town is any citadel town built upon hills to make invasion difficult. Often protected by defensive walls, steep embankments, or cliffs, such hilltop settlements provided natural defenses for their inhabitants. In Europe, especially in Italy, Spain, Portugal and southern France, such towns were common. Around the world, the most famous examples are the hilltowns of Darjeeling and Simla. In recent years, Bill Buchanan, Douglas Duany, Lucien Steil and others have studied hill towns with an interest in reviving interest in the enduring form.