Background
He was the second son of Mieszko I, Duke of Cieszyn by his wife, probably called Grzymisława. After death of his father in 1315, Casimir I gained the southwestern part of the duchy, centered around the town of Cieszyn.
Duke of Cieszyn Duke of Siewierz
He was the second son of Mieszko I, Duke of Cieszyn by his wife, probably called Grzymisława. After death of his father in 1315, Casimir I gained the southwestern part of the duchy, centered around the town of Cieszyn.
Casimir I initially had good relations with Władysław I the Elbow-high, who became the King of Poland in 1320. But when during 1321–1324 Lithuanian forces supporting Elbow-high plundered Cieszyn, Casimir I broke with the Polish King. Fifteen days later, on 23 February, he received Cieszyn as a hereditary possession, maintaining a large internal sovereignty.
When Duke Leszek of Racibórz died in 1336, Casimir I tried unsuccessfully to obtain his lands.
King John of Bohemia granted this land to Duke Nicholas II of Opava. In 1355, after the death of Bolesław, Duke of Koźle-Bytom without male issue, Casimir I entered a conflict over his inheritance with Konrad I of Oleśnica.
The dispute was only resolved in 1357 (the decision was made as early as 1355, but Bytom remained until that time under the rule of Margareta of Sternberg —Bolesław"s widow— as her Oprawa wdowia): the Duke of Cieszyn took possession of half of Bytom and Gliwice, Toszek and Pyskowice. In the internal politics, Casimir I supported the economic development of his Duchy, mostly towns.
He also surrounded Cieszyn with walls and by 1320 granted the town of Bielsko with German laws.
Casimir I died in 1358 (although there are some indications that he lived until 1360, but this is refuted by the majority of historians) and was buried in the Dominican church of Cieszyn.