During the Middle Ages, Thuringia was situated at the border between Germanic and Slavic territories, marked by the Saale river. The Ostsiedlung movement led to the assimilation of Slavic people between the 11th and the 13th century under German rule. The population growth increased during the 18th century and stayed high until World War I, before it slowed within the 20th century and changed to a decline since 1990. Since the beginning of Urbanisation around 1840, the Thuringian cities have higher growth rates resp. smaller rates of decline than rural areas (many villages lost half of their population since 1950, whereas the biggest cities (Erfurt and Jena) keep growing).