In 1310 he offered some of Masovia to his son (the Duchy of Warsaw and the Duchy of Czersk).
By 1321, Casimir I married Euphemia (b. 1310 – d. aft. 11 January 1374), daughter of Duke Trojden I of Czersk-Warsaw.
Starting from 1990 – Systematic development of modern infrastructure (potable water system, sewer system and wastewater treatment plants (Czersk and Rytel), heating gas distribution, development of modern road system, modernisation of railroad);
While Siemowit's son Duke Konrad II (1264–1294) moved his residence to Czersk he and his brother Boleslaus II entered into a long-term conflict over the Polish seniorate with their Kuyavian relatives and the Silesian Piasts, which estranged them from the Piast monarchy.
She was the fourth daughter and youngest child of Casimir I, Duke of Cieszyn, by his wife Euphemia, daughter of Duke Trojden I of Czersk-Warsaw.
In 1374 he inherited Zakroczym and in 1381 the lands of Czersk, Liw and Wyszogród as the result of partition of Masovia between his younger brother Siemowit IV and himself.
Originally, their main settlements were in the area of Płock, Łomża, Wizna, Czersk, Ciechanów, Płońsk, Zakroczym and Grójec.
Different capitals of individual former duchies of Mazovia also include Czersk and later Warsaw.
In 1310, when his father was still alive Trojden received the small region Czersk.
Czersk | Czersk, Masovian Voivodeship | Czersk Operational Group | Operational Group "Czersk" |
Kurkowo, Pomeranian Voivodeship, a village in the administrative district of Gmina Czersk, Poland
# Operational Group "Czersk" (Grupa Operacyjna Czersk) under gen. Stanisław Grzmot-Skotnicki
Siemowit gave the small region of Warsaw to his brother Casimir, retaining the regions of Czersk, Liw and Rawa.
At the start of the Invasion of Poland in 1939, he commanded his unit as the core of the Czersk Operational Group which was to shield the Vistula river crossings against the German offensive and to protect the flanks of the Pomorze Army.