asked the Knights to protect him from retaliatory raids by the Prussians. Preoccupied with crusades in the Holy Land, the Teutonic Knights arrived only in 1230. Their first task was to build a base on the left bank of Vistula at Vogelsang, opposite of Toruń (Thorn), which was completed a year later. Led by Hermann Balk, the Knights did not repeat the mistakes of the previous Order and did not push eastwards into the forest of the interior. They would further build fortified log (later brick and stone) castles along major rivers and the Vistula Lagoon to serve as basis for future expansion. In 1231–1242, forty such castles were built. The Prussians faced major difficulties in capturing these castles as they were accustomed only to combat in open fields. Most conflicts occurred either in summer or winter. Heavily armoured knights could not travel and fight on land soaked by water from melting snow or autumn rains. Summer campaigns were most dangerous as the Knights would immediately build new castles in the conquered territory. The Teutonic Knight's strategy proved successful: in ten years, five of the seven major Prussian clans fell under control of the less-numerous Teutonic Knights. However, the Prussians further resisted the conquerors, leading to five uprisings over the following fifty years. The first Prussian uprising (1242–1249) The first Prussian uprising was influenced by three major events. Firstly, the Livonian Knights – a subsidiary of the Teutonic Knights – lost the Battle of the Ice on Lake Peipus to Alexander Nevsky in April 1242. Secondly, southern Poland was devastated by a Mongol invasion in 1241; Poland lost the Battle of Legnica and the Teutonic Knights lost one of its most trusted allies that often supplied troops. Thirdly, Duke Swantopolk II of Pomerania was fighting against the Knights, who supported his brothers' dynastic claims against him. It has been implied that the new castles of the Knights were competing with his lands over the trade routes along the Vistula River. While some historians embrace the Swantopolk–Prussian alliance without hesitation, others are more careful. They point out that the historical information came from documents written by the Teutonic Knights and must have been ideologically charged to persuade the Pope to declare a crusade not only against the pagan Prussians but also against the Christian duke. Prussians besieged Teutonic castles and managed to capture all except for Elbing (Elbląg) and Balga in