the Ottomans installed Bethlen as Prince of Transylvania and this was endorsed on 13 October 1613 by the Transylvanian Diet at Kolozsvár (Cluj-Napoca). In 1615, after the Peace of Tyrnau, Bethlen was recognised by Matthias, Holy Roman Emperor. Bethlen's rule was one of enlightened absolutism. He developed mines and industry and nationalised many branches of Transylvania's foreign trade. His agents bought goods at fixed prices and sold them abroad at profit. In his capital, in Gyulafehérvár (Alba Iulia), Bethlen built a grand new palace. Bethlen was a patron of the arts and the Calvinist church, giving hereditary nobility to Protestant priests. Bethlen also encouraged learning by founding the Bethlen Gabor College, encouraging the enrollment of Hungarian academics and teachers and sending Transylvanian students to the Protestant universities of England, the Dutch Republic, and the Protestant principalities of Germany. He also ensured the right of serfs' children to be educated. Anti-Habsburg insurrection Bethlen maintained an efficient standing army of mercenaries. While keeping relations with the Sublime Porte (the Ottoman Empire), he sought to gain lands to the north and west. During the Thirty Years' War, he attacked the Habsburgs of Royal Hungary (1619–1626). Bethlen opposed the autocracy of the Habsburgs; persecution of Protestants in Royal Hungary; the violation of the Peace of Vienna of 1606; and Habsburg alliances with the Ottomans and George Drugeth (1633-1661), the captain of Upper Hungary. In August 1619, Bethlen invaded Royal Hungary. In September, he took Kassa (Košice) where Protestant supporters declared him the leader of Hungary and protector of Protestants. He gained control of Upper Hungary (present-day Slovakia). In September 1619, after refusing to convert to Calvinism, the Jesuits Marko Križevcanin, Stephen Pongracz and Melchior Grodeczki were martyred under Bethlen's authority." The three were later canonized by the Catholic Church. In October 1619, Bethlen took Pressburg (Pozsony, today's Bratislava), where the Palatine of Hungary ceded the Holy Crown of Hungary. However, Bethlen, together with Jindřich Matyáš of Thurn-Valsassina, count of the Moravian and Czech estates, did not take Vienna and, in November, the forces of George Drugeth and Polish mercenaries (lisowczycy) won the Battle of Humenné and forced Bethlen to leave Austria and Upper Hungary. Bethlen negotiated for peace at Pressburg, Kassa (now Košice) and Besztercebánya (now Banská Bystrica). In January 1620, without the Czechs, Bethlen received 13 counties in the east of Royal Hungary. On 20 August 1620, he was elected King of Hungary