On 23 April 1551, at the age of sixteen, William was declared an adult by a decree of Emperor Ferdinand I. He took up the administration of the family's possessions and chose as his residence Český Krumlov Castle, which he remodeled in a Renaissance style. From 1552 to 1556, he fought a court case against Henry IV of Plauen, the High Chancellor of Bohemia, about their precedence in Bohemian society. William won. In 1560, he was appointed High Treasurer by King Ferdinand. In 1566, he was appointed commander of the Bohemian army and on 10 June 1566, he began forming an army at Znojmo to fight a war against the Ottoman Empire. Their aim was to recapture Szigetvár, which William's brother-in-law Nikola Šubić Zrinski had lost to the Turks. The Habsburg armies were to meet at Győr. However, the Turks retreated south after the death of Suleiman the Magnificent and no battle ensued.