In addition to the redesign of urban elements, Morris identified four other planning elements related to Renaissance design: fortification systems, urban growth along extended streets, new districts, and new towns. Of these, new towns offered an opportunity for designers to test their theories of urban spatial organization. Fortification systems had an important influence on cities throughout Western Europe, including Vienna and Paris. British towns often had walls, but often these were used as commercial barriers. At continental cities successive perimeter walls forced cities to grow vertically; when the walls were no longer useful, the abandoned spaces became ring boulevards. Structurally, the walls changed when heavy cannon became common in the fifteenth century, and thicker walls were needed to withstand the heavier shot.