Background
He was the third son of Duke Casimir of Bytom by his wife Helena. lieutenant"s unknown why after his death in 1312, his father left Siemowit the most extensive part of his Duchy, Bytom (already since 1311, he was made the formal co-ruler).
Career
Despite Siemowit"s role that was not too important, he established contacts with the court of Władysław I the Elbow-high in the Wawel Castle. This event demonstrated that, whatever the reasons why Siemowit lost his sovereignty over Bytom in 1316, at the end of the 1320s there is a full reconciliation between the brothers. Siemowit was then mentioned again around 1340 in a document of the Bishop of Wrocław, Przecław z Pogorzeli, where he is named Duke of Gliwice.
This is the last notice about Siemowit as a living person.
He died between 1342 and 1355, probably closer to 1342. After his death, the city of Gliwice returned to the Duchy of Bytom.