In disputes prior to the Reformation of 1536, the city which had been faithful to Christian II, who was Catholic, was successfully besieged in 1523 by the forces of Frederik I, who supported Lutheranism. Copenhagen's defences were reinforced with a series of towers along the city wall. After an extended siege from July 1535 to July 1536, during which the city supported Christian II's alliance with Malmö and Lübeck, it was finally forced to capitulate to Christian III. During the second half of the century, the city prospered from increased trade across the Baltic supported by Dutch shipping. Christoffer Valkendorff, a high-ranking statesman, defended the city's interests and contributed to its development. The Netherlands had also become primarily Protestant, as were northern German states.