Siemowit I (1215-1262)
Prince of Masovia and Czersk, son of Masovian Prince Konrad and Agafia of the Rurik dynasty. His wife was Pereyslawa, daughter of Daniel – Prince of Halicz and Włodzimierz. In the years 1246–1248, he was allowed to govern the land of Czersk, after which he was given power over the Czersk District.
In 1248, after the death of Bolesław I, he took charge of the Duchy of Masovia. He participated in several sorties against the Yotvingians. He made several treaties with the Teutonic Order. They allowed him to own some Baltic lands and defend from attacks of the tribes of pagan Balts.
During his reign, the Duchy of Masovia was threatened by the rising power of Lithuania. In 1262, when he stayed in the Jazdów borough (future Ujazdów), he was assaulted by Lithuanian armies. Taken into captivity, he was immediately beheaded, and his remains were burned. In the gardens of the Royal Łazienki – the presumed location of the borough of Jazdów – stands a column crowned with a sculpture of an eagle.