Conrad had built a small castle for the knights, called Vogelsang, on the west bank of the Vistula, in which they placed a modest garrison. In 1228 attempts were made to organize a local, Polish knightly order for defence against the Prussians. It was called 'the Dorbrzyn Brothers,' under the auspices of the abbey of Szepatow near Wloclawek and Bishop Christian. They were granted the lands between the Vistula and the rivers Skrwa and Drweca.
The Knights received from Conrad the region of Chelmno, but only as vassals of the prince. They also concluded an agreement with Bishop Christian and started operations, moving down the Vistula. They crossed it in the following year and built in the vicinity of the present town of Torun a small fort, which had a guardhouse in the branches of a giant oak tree.
One of their first victims was a man by the name of Pipin, who owned the small castle of Piegza. The armed monks captured him and punished him in their own way for reverting to paganism after having been baptised earlier. They cut open his abdomen, nailed on end of his entrails to a tree and chased him around it until his intestines were wrapped round the trunk.
Such practices were not uncommon in the conquest of Prussia by the Knights of the Cross.
In 1234 Bishop Christian was captured by the Prussians. The Knights used the opportunity to submit to the pope a forged document by which Conrad of Masovia was supposed to have granted them in perpetuity the region of Chelmno and all of Prussia. The pope declared that country 'the property of Saint Peter' and granted it to the Knights. A few years later he recognized Prussia as his fief, thus giving legal standing to the state of the Knights of the Cross.
Expeditions were systematically sent out and after defeating the natives the Knights built a castle in a key strategic location. The Prussians could not take it and therefore had to surrender. Systematic colonization was started. Kwidzyn was founded in 1234 and Elblag three years later, allowing the Germans to bring in reinforcements by sea. In 1237 the German Knights of the Sword of the Dzwina region in the north joined forced with the Knights of the Cross, thus encircling the Baltic countries.
The conquest of the rest of Prussia did not take long. Striking simultaneously from west and east, the Germans tooks the Kuron Bight in the years 1252-1254 and founded Memelburg. In 1255 the king of Bohemia, Ottokar II, came to help the Knights. Their joint expedition reached the river Pregola and the town founded their was called Konigsberg in honor of the royal guest.
The conquest of Prussia was concluded in 1283, half a century after the first detachments of the Knights had crossed the western border of that country under the regional Master Herman von Balk. To reinforce the new German state, the Order brought from Germany and settled on land grants not only knights, but also burghers and peasants. The Prussian peasants, brutally persecuted, were enslaved or exterminated. In the initial stage of the German occupation, there were even forbidden to marry.