Renowned Polish astronomer Nicolaus Copernicus lived in the prince-bishopric, in the towns of Lidzbark, Olsztyn and Frombork, and there he began and completed his groundbreaking work on heliocentrism. In 1521 Copernicus commanded the successful Polish defense of Olsztyn during a siege by the Teutonic Knights during the Polish–Teutonic War (1519–1521).