century the water castle was converted into a manor house (Schloss). But it was not big enough to host the court, and was not surrounded by a larger settlement. So it was never a palace, but more of a manor house and was only the seat of the advocacy for a limited time. The actual administrative centre of the Principality was in Neustadt. The fortress of Calenberg was, however, regarded by the princes as their ancestral home and was looked after accordingly. In 1634, after a number of divisions of the estate, the principalities of Calenberg and Göttingen were merged into the Principality of Brunswick-Lüneburg-Celle and, after the granting of electorate status in 1692, became part of the Electorate of Hanover. In 1636, Duke George of Brunswick-Calenberg recaptured the territory for the House of Welf during the Thirty Years with the help of Swedish troops, and chose Hanover as a residence. From 1648 the manor house itself was turned back into a fortress. It then served as a garrison, from 1673 as a glass factory and then as a prison. The fortress was repaired again from 1656 to 1662. On the left bank of the river a demesne was built as seat for the administration of the Amt as well as of the castle's farmlands. Imprisonment of Corvinus The Catholic duke, Eric II accepted in 1548 the Augsburg Interim from the Catholic Emperor. At the Lutheran synod of Münden in 1549, the Lutheran reformer, Anton Corvinus, and the Pattensen preacher, Walter Hoiker, (also Hocker), together with 140 priests, voiced their bitter opposition to the Interim. As a result, on 2 November 1549, Eric II placed Anton Corvinus and Walter Hoiker in custody in Calenberg Fortress for contempt to force them and the other clergy to accept the Interim. The prisoners were well cared for, could receive and reply to letters and talk to visitors through an open window. After the Peace of Passau, when the Emperor declared the Interim invalid and Eric II had fallen out of favour with him, the two prisoners were released on 21 October 1552. Merian copperplate In 1654 Caspar Merian (1627–1686) published his copperplate of Calenberg Fortress in Topographia Germaniae Braunschweig-Lüneburg, which was based on a sketch by survey engineer, Conrad Buno. The copperplate shows a perspective view from the village of Gestorf towards Hildesheim, i.e. from northwest to southeast. In the left foreground stand