Another contemporary tendency is the conversion of industrial buildings into mixed residential communities. An example is the Gasometer in Vienna, a group of four massive brick gas production towers constructed at the end of the 19th century. They have been transformed into a mixed residential, office and commercial complex, completed between 1999 and 2001. Some residences are located inside the towers, and others are in new buildings attached to them. The upper floors are devoted to housing units the middle floors to offices, and the ground floors to entertainment and shopping malls. with sky bridges connecting the shopping mall levels. Each tower was built by a prominent architect the participants were Jean Nouvel, Coop Himmelblau, Manfred Wehldorn and Wilhelm Holzbauer. The historic exterior walls of the towers were preserved.