During the 16th century, work began to turn the castle into a fortress that could withstand the new gunpowder weapons. One of the first cannon was the 9 ton Greif cannon. At the foot of the hill, protected by the fortress, Philipp Christoph von Sötern had the palace "Phillipsburg" constructed in 1625–1629. The fortress was further improved by his successors Karl Kaspar von der Leyen and Johann Hugo von Orsbeck. In 1632, it was occupied by the French and in 1637 by Imperial troops during the Thirty Years' War.