In 1709, Dorotheenstadt was united with the two cities of Berlin and Cölln and the other two electoral 'new towns' of Friedrichswerder and Friedrichstadt to form the "royal capital and residence city" of Berlin. As the 18th century continued, it was extended to the Spree in the north and the Tiergarten in the west. Originally the new towns were overwhelmingly residential, filled with stone houses 2 or 3 storeys in height, but in the second half of the 19th century, large government and business buildings replaced them. The eastern part of Dorotheenstadt, near the museums, developed into an academic area, including the Berlin University (now Humboldt University) and the Berlin State Library. Following the construction of the Berlin Stadtbahn in 1880, a vibrant urban centre grew up around Friedrichstraße station alongside the desirable and elegant district of Unter den Linden. The population fell from 20,144 in 1867 to 11,492 in 1910, as the area became less residential. In 1920, with the creation of Greater Berlin, Dorotheenstadt was incorporated into the newly created borough of Mitte.