Lilienthal, Lower Saxony conversion of the Cistercian nuns, it became a Lutheran Women's Convent, and until the end of the Thirty Years' War in 1648 it remained in deplorable conditions. In 1650, the city went through secularization. The land once belonging to the convent, which had become fragmented, developed into the small town of Lilienthal. In 1648, according to the terms of the peace treaty that ended the Thirty Years' War (The Peace of Westphalia), the Prince-Archbishopric was transformed into the Duchy of Bremen, ruled together with the Principality of Verden, which were first ruled in personal union by the Swedish Crown. Queen
Hessian War Peace Congress mediated by Duke Ernest of Saxe-Gotha and which resulted in a unification and peace treaty that was sealed in April 1648, ahead of the Treaty of Westphalia itself. Before the Westphalian Peace Treaty, Cassel fought one last time against the imperial side, winning the Battle of Wevelinghoven in the Rhineland, together with other Protestant troops. Darmstadt, however, was not involved in this battle. Under the Unification Treaty between Cassel and Darmstadt, Upper Hesse was permanently divided. Darmstadt had to give up a significant part of Upper Hesse to Cassel, not least Marburg and other occupied territories, including the