Eric I, Calenberg Castle was expanded into a strong fortress. Another heavily fortified castle, which he had built, was the Erichsburg near Dassel on which construction began in 1527. In the Hildesheim Diocesan Feud in 1519 he was initially defeated militarily in the Battle of Soltau. Diplomatically, however, he was able to win a ruling from the Emperor Charles V that saw a large part of the Bishopric of Hildesheim added to his domain. Eric I was hostile to the emerging Protestant Reformation. His second wife, Elisabeth of Brandenburg, however, whom he married in 1525, switched over to the new doctrine in 1535 and promoted it at the court, which then resided at Münden. After Eric's death in 1540 she took over the government for their underage son, Eric II, and implemented the Reformation in the Principality with the state superintendent Antonius Corvinus she had appointed. Eric II, however, converted to Roman Catholicism in 1547 even though he was raised as an Evangelical by his mother. He was not able to reverse the Reformation in the Principality however. His power in the principality was already very weak. He spent most of his time as a mercenary leader abroad, and was financially dependent on the towns. In 1553 he had to secure the financial aid of his towns by approving evangelical preaching. From 1574 he had Neustadt am Rübenberge developed as a fortified town and built Landestrost Castle within its walls as a Renaissance chateau, integrated into a bastion fortress based on the Italian model. In 1582 when the counts of Hoya died out, the larger part of the county went to Calenberg. In 1584 Calenberg also acquired the Diepholz. Thirty Years' War After Eric's death in 1584 Calenberg-Göttingen was again ruled by the Wolfenbüttel line of the Welfs. In the Thirty Years' War the brother of Duke Frederick Ulrich, "mad" Christian, brought the war to the state. After Danish troops under King Christian IV, who was then commander of the Lower Saxon Circle, was defeated by the general of the Catholic League, Tilly in the Battle of Lutter, Tilly occupied the whole principality in 1626. Only the cities of Brunswick and Hanover could not be captured. When Duke Frederick Ulrich died childless in 1634 the Wolfenbüttel line of the Middle House of Brunswick ended with him. In 1635 Duke Augustus the Elder from the Middle House of Lüneburg received the