Complex sentence: Under their governance, woodlands were cleared and marshlands made arable, upon which many cities and villages were founded, including Marienburg (Malbork) and Königsberg (Kaliningrad).
Split into simpler sentences: Under their governance, woodlands were cleared and marshlands made arable. Many cities and villages were founded upon those lands, including Marienburg (Malbork) and Königsberg (Kaliningrad).