concern of Béla's policy. In a letter of 1247 to Pope Innocent IV, Béla announced his plan to strengthen the Danube—the "river of confrontations"—with new forts. He abandoned the ancient royal prerogative to build and own castles, promoting the erection of nearly 100 new fortresses by the end of his reign. These fortresses included a new castle Béla had built at Nagysáros (Veľký Šariš, Slovakia), and another castle Béla and his wife had built at Visegrád. Béla attempted to increase the number of the soldiers and to improve their equipment. He made land grants in the forested regions and obliged the new landowners to equip heavily armoured cavalrymen to serve in the royal army. For instance, the so-called ten-lanced nobles of Szepes (Spiš, Slovakia) received their privileges from Béla in 1243. He even allowed the barons and prelates to employ armed noblemen, who had previously been directly subordinated to the sovereign, in their private retinue (banderium). Béla granted the Banate of Szörény to the Knights Hospitaller on 2 June 1247, but the Knights abandoned the region by 1260. To replace the loss of at least 15 percent of the population, who perished during the Mongol invasion and the ensuing famine, Béla promoted colonization. He granted special liberties to the colonists, including personal freedom and favorable tax treatment. Germans, Moravians, Poles, Ruthenians and other "guests" arrived from neighboring countries and were settled in depopulated or sparsely populated regions. He also persuaded the Cumans, who had in 1241 left Hungary, to return and settle in the plains along the River Tisza. He even arranged the engagement of his firstborn son, Stephen, who was crowned king-junior in or before 1246, to Elisabeth, a daughter of a Cuman chieftain. Béla granted the privileges of Székesfehérvár to more than 20 settlements, promoting their development into self-governing towns. The liberties of the mining towns in Upper Hungary were also spelled out in Béla's reign. For defensive purposes, he moved the citizens of Pest to a hill on the opposite side of the Danube in 1248. Within two decades their new fortified town, Buda, became the most important center of commerce in Hungary. Béla also granted privileges to Gradec, the fortified center of Zagreb, in 1242 and confirmed them in 1266. Béla adopted an active foreign policy soon after the withdrawal of the Mongols. In the second half of 1242 he invaded Austria and forced Duke Frederick