consult Diets in Hungary with more frequency than his predecessors and institute the office of Palatine as chief administrator while he was away. In 1404, Sigismund introduced the Placetum Regium. According to this decree, Papal bulls could not be pronounced in Hungary without the consent of the king. During his long reign, the royal castle of Buda became probably the largest Gothic palace of the Late Middle Ages. King of Croatia In about 1406, Sigismund married Mary's cousin Barbara of Celje, daughter of Count Hermann II of Celje. Hermann's mother Catherine (of the House of Kotromanic) and Mary's mother Queen Elisabeth of Bosnia were sisters, or at least cousins who were adoptive sisters. Sigismund managed to establish control in Slavonia. He did not hesitate to use violent methods (see Bloody Sabor of Križevci), but from the river Sava to the south his control was weak. Sigismund personally led an army of almost 50,000 "crusaders" against the Bosnians, culminating with the Battle of Dobor in 1408, a massacre of about 200 noble families. Possessions in Serbia Threatened by Ottoman expansion, king Sigismund managed to strengthen the security of southern Hungarian borders by entering into a defensive alliance with despot Stefan Lazarević of Serbia. In 1403, Hungarian possessions in northwestern regions of Serbia (city of Belgrade and the Banate of Macsó), were given to despot Stefan, who pledged his allegiance to king Sigismund, remaining the king's loyal vassal until death in 1427. Stefan's successor George Branković of Serbia also pledged his allegiance to Sigismund, returning Belgrade to the king. By maintaining close relations with Serbian rulers, Sigismund succeeded in securing southern borders of his realm. Order of the Dragon Sigismund founded his personal order of knights, the Order of the Dragon, after the victory at Dobor. The main goal of the order was fighting the Ottoman Empire. Members of the order were mostly his political allies and supporters. The main members of the order were Sigismund's close allies Nicholas II Garay, Hermann II of Celje, Stibor of Stiboricz, and Pippo Spano. The most important European monarchs became members of the order. He encouraged international trade by abolishing internal duties, regulating tariffs on foreign goods and standardizing weights and measures throughout the country. King of the Romans After the death of King Rupert of Germany in 1410, Sigismund – ignoring the claims of his half-brother Wenceslaus – was elected as successor by three