The 15th century brought a new dimension to the history of Kadaň: the town and castle often used to be pledged to royal creditors. At the end of the rule of George of Poděbrady first the town and later the castle was captured by Jan Hasištejnský z Lobkovic (John of Lobkowicz and Hassenstein), who seemed to see Kadaň as a suitable capital for his dependent territories. The establishment and construction of the monastery of the Franciscans of strict observance, built as a family burial-place and later a pardon place of Fourteen Holy Helpers, may reflect his ambitions for the town. Jan Hasištejnský died in 1517 and is buried in the Church of Fourteen Holy Helpers. His tomb is still on display in the Church, now part of the municipal museum. The same year saw the beginning of the Reformation in Germany, which had an immediate impact on the atmosphere in German speaking Kadaň. In 1534, "Kadaň religious peace" was negotiated here between Württemberg Protestants and Ferdinand I, Holy Roman Emperor for the Catholic side. In the Thirty Years' War during the next century, Kadaň suffered from fires and plundering by various armies on their way to the battlefields of Bohemia. After the war, the formerly Czech-German town became a monolingual German domain.