Task: Question Generation
Input (English): Passage: The first reinforcement to the Teutonic forces arrived in early 1261, but was wiped out on January 21, 1261 by Herkus Monte in the Battle of Pokarwis. In January 1262 reinforcements arrived from the Rhineland, led by Wilhelm VII, Duke of Jülich, who was obliged by Pope Alexander IV to fulfil his crusader duties in Prussia. This army broke the Siege of Königsberg but as soon as the army returned home, the Sambians resumed the siege and were reinforced by Herkus Monte and his Natangians. Herkus was later injured and the Natangians retreated, leaving the Sambians unable to stop supplies reaching the castle and the siege eventually failed. Prussians were more successful capturing castles deeper into the Prussian territory , and the Knights were left only with strongholds in Balga, Elbing, Culm, Thorn, and Königsberg. Most castles fell in 1262-1263, and Bartenstein fell in 1264. The Prussians destroyed captured forts instead of using them for their own defence, so the end of successful sieges meant that large Prussian forces did not have to stay near their home and were then free to operate in other parts of Prussia, raiding the Culmerland and Kuyavia. A recovered Herkus Monte raided Culmerland with a large force and took many prisoners in 1263. On his way back to Natangia, Herkus and his men were confronted by a contingent of their enemies. In the Battle of Löbau that ensued, Prussians killed forty knights, including the Master and the Marshal. The Prussians also received help from Lithuanians and Sudovians. In summer of 1262 Treniota and Shvarn attacked Masovia, killing Duke Siemowit I, and raided Culmerland, provoking Pogesanians to join the uprising. However, assassination of Mindaugas and subsequent dynastic fights prevented Lithuanians from further campaigns. Skalmantas, leader of Sudovians, raided Culm  in 1263 and in 1265.
Output (English): How many strongholds were the Knights left with in Prussia?