A body of these "uskoks" led by Croatian captain Petar Kružić used the base at Klis both to hold the Turks at bay, and to engage in marauding and piracy against coastal shipping. Although nominally accepting the sovereignty of the Habsburg Emperor Ferdinand I, who obtained the Croatian crown in 1527, Kružić and his freebooting Uskoks were a law unto themselves. After Petar Kružić's death, and the lack of water supply, the defenders of Klis finally surrendered to the Ottomans in exchange for their freedom. On March 12, 1537, the town and fortress was given up to the Ottomans, many of the citizens left the town while the Uskoks went to the city of Senj on the Adriatic coast, where they continued fighting the Ottomans. They may have started to gather around Senj as early as 1520 (Bracewell 1992). The Ottoman raids and destruction brought Senj natives together with those from the Habsburg lands, Dalmatians, Ragusans and Italians. At Senj, the Klis Uskoks were soon joined by other refugees from Novi Vinodolski in northwestern Croatia, from Otočac on the Gacka River, and from other Croatian towns and villages. The new Uskok stronghold, screened by mountains and forests, was unassailable by cavalry or artillery. However, the fortress was admirably suitable to the lightly armed uskoks who were excellent in guerrilla warfare. The Martolos were employed by the Ottomans to discourage Uskok penetration of Turkish territory, which was not very profitable anyhow. Since the uskoks were checked on land and were rarely paid their annual subsidy, they resorted to acts of piracy. Large galleys could not anchor in the bay of Senj, which is shallow and exposed to sudden gales. So, the uskoks fitted out a fleet of swift boats, which were light enough to navigate the smallest creeks and inlets of the shores of Illyria. Moreover, these boats were helpful in providing the uskoks a temporary landing on shore. With these they were able to attack numerous commercial areas on the Adriatic. The uskoks saw their ranks swell as outlaws from all nations joined them. Eventually, the whole city of Senj lived from piracy. The expeditions were blessed in the local church and the monasteries of the Dominicans and the Franciscans received tithes from the loot. After the War of the Holy League in 1537 against the Ottoman Empire, a truce between Venice and the Ottomans was created in 1539. This