The von Saldern family had to relinquish the castle in 1364 to Duke William because they had participated in a secret pact with the Bishop of Hildesheim. In 1380 Calenberg Castle withstood a siege by the Bishop of Hildesheim; after which the episcopal castle of Nabershausen near Barnten was slighted. In 1371 the castle became the seat of the ducal advocates (Vögte) in the 'Grand Advocacy' (Großvogtei) of Calenberg and, in 1432, it was named as a Residenz for one of the Welf dukes. From 1405 the castle became the administrative seat of the Advocacy (Vogtei) of Calenberg. Fortress Following the introduction of firearms the moated castle was no longer capable of resisting attack. At the beginning of the 16th century, and before the Hildesheim Diocesan Feud, it was therefore rebuilt under Eric I into a modern fortress, accessible over two bridges. The most important measure was the construction of a 700 m long wall, which was to protect the castle square from gunfire. The main wall with eight stone sconces was surrounded by a 40 m wide moat, which was flooded by a channel from the Rössingbach stream. In front of the western entrance with its drawbridge, an unfortified 'island' (Vorwerksinsel) was laid out and given the field name Die Bleiche. Directly behind the bridge was the three-story battery tower, about 24 m in diameter with 2 m thick walls and gun slits arranged in pairs. The fort was defended by seventeen guns and was thus better equipped than the towns of Göttingen and Hanover that belonged to the Principality. The fortress of Calenberg survived several sieges. During the Great Diocesan Feud in 1519 it was besieged for three weeks without success. In the Thirty Years War the fortress of Calenberg was besieged in 1625 for three weeks by Tilly and it was surrendered only after a mutiny by the troops. In 1632 the Welf duke, George of Brunswick-Calenberg, fighting on the Swedish side, did not initially succeed in retaking the castle despite a six-week siege. Only after his victory in the Battle of Hessisch-Oldendorf and another siege, was he able to capture the heavily damaged fortress of Calenberg in 1633. Later Usage The fortress helped to reduce the influence of the bishops of Hildesheim, with whom the Welfs lived in constant conflict, in the region of Hanover and to achieve territorial gains. At the start of the 16th