During the period of the formation of early - Piast  country, reinforced tribal borough became an important borderline  guardhouse on the western external borders of Poland. The borough faced German invasions on Poland during rules of Henry I the Bearded (1202 – 1238). In those days, Krosno  again became weighty strategic point and administration and economic centre. Henry I the Bearded reformed Krosno spatially  and awarded the Magdeburg rights, he also contributed to rural colonization development, which during his rules took the form of close settlements rationally arranged. In the middle of the XIII century, there came into being the Duchy of Głogów,  which includes the land of Krosno Castellany as well . It bordered then with Brandenburg estates. As Magdeburg was a part of Brandenburg those days, Lubusz Land was sold by  Bolesław II the Rogatka  to Brandenburg in 1249.

In 1319 the above mentioned area came back to the Dutchy, and next ruling over Krosno Dukes were successively  Głogów and Żagań Henries IV, V, VI, VIII, IX and  XI. The Dukes resided in Żagań or in Kożuchów.  Henry XI was married to the daughter of the last Elector of Brandenburg, Barbara. After his  death, Krosno became the widow's place of residence under Brandenburg control. The close relative of dead Henry XI – Jan II the Mad (former Duke of Żagań) couldn't reconcile to such situation, as he had been fighting with  Brandenburg about recovery of his heritage for six years ( in the years 1476 – 1482). Upon the decision of peace in Kamieniec, Krosno remained under rules of  Brandenburg. However from newly conquered lands, which included also Sulechów and Lubusz Districts,  there was established the new Dutchy of Krosno. In 1518,  there took place the personal union between Brandenburg and Royal Prussia, and in the first half of XVIII century Brandenburg became the province of the Kingdom of Prussia. After the war with Austria and joining Silesia to Prussia in 1740, there carried out the reform, thus dividing the country into districts. There was formed  Krosno District covering the western part of former Castellany  and some part of Lubusz District. Such division lasted till 1945.