Austrian ethnographers speak of six identifiable rural settlement forms: (1) single, isolated farms with field blocks; (2) hamlets with tenant holdings; (3) nucleated villages with strip fields; (4) linear villages with strip fields extending through wooded areas; (5) villages built around a central green with rationalized fields; and (6) villages built along a street with rationalized fields. The more diffuse settlements (types 1,2, and 4) are found in alpine zones. The more nucleated settlements (types 3, 5, and 6) are found in lowland areas. Urban settlements are primarily riverine, nucleated, and, originally, walled. These features derive from the early modern period of town formation (1350-1650) in central Europe when waterways were used as transport routes and there was a high level of political and military insecurity. The most important regional centers—Innsbruck (Tirol), Salzburg (Salzburg), Linz (Upper Austria), Villach and Klagenfurt (Carinthia), Eisenstadt (Burgenland), Graz (Steiermark), and Saint Polten and Wiener Neustadt (Lower Austria)—are of this type. Vienna, with 20 percent of the national population, is a world-class metropolis and a center for elite entertainments and tourism. It was originally a Roman frontier fortress (Vindobona, A.D. 140) that declined in the post-Roman period only to revive with the building of Saint Stephan' s Cathedral in the twelfth century. It was a center of commerce in the early modern period, when it enjoyed staple rights over traffic up and down the Danube. Economic Development stagnated in the seventeenth and eighteenth centuries as the Habsburg dynasty transformed the city into a Ceremonial and administrative center for the empire. The Industrial transformation of the metropolis began late (1820s) and proceeded at a leisurely pace. The razing of the city's walls in 1857 and the development of the broad Ring Boulevard around the central district heralded the beginning of modern city government and planning. By the mid-1890s, all but two of the current twenty-three districts had been annexed from previously autonomous suburbs and the population had swollen to its historic high of 2 million people, two-thirds of whom had been born elsewhere and migrated to the city for industrial employment.