 By the 13th century the territory was subject to raids by pagan Old Prussians, who sacked Chełmno, the province's main town, in 1216.In 1220 Conrad I of Masovia, with the participation of the other dukes of Poland, led a partial reconquest of the province, but the project of establishing a Polish defense of the province failed due to conflicts between the dukes.He brought the crusading Knights of Dobrzyń to Masovia, where they built a castle at Dobrzyń in 1224 as a base for attacks against the Prussians.