In 1539, a 60,000-strong Ottoman army besieged the Spanish garrison of Castelnuovo on the Adriatic coast; the successful siege cost the Ottomans 8,000 casualties, but Venice agreed to terms in 1540, surrendering most of its empire in the Aegean and the Morea. France and the Ottoman Empire, united by mutual opposition to Habsburg rule, became strong allies. The French conquests of Nice (1543) and Corsica (1553) occurred as a joint venture between the forces of the French king Francis I and Suleiman, and were commanded by the Ottoman admirals Barbarossa Hayreddin Pasha and Turgut Reis. A month before the siege of Nice, France supported the Ottomans with an artillery unit during the 1543 Ottoman conquest of Esztergom in northern Hungary. After further advances by the Turks, the Habsburg ruler Ferdinand officially recognised Ottoman ascendancy in Hungary in 1547. Suleiman I died of natural causes in his tent during the Siege of Szigetvár in 1566.