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When Casimir III the Great died in 1370, under agreements concluded with one of his lifetime friends Siemowit III, he freed Siemowit from Polish suzerainty and gave Płock, Wizna, Wyszogród and Zakroczym back to Mazovia.
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Siemowit gave the small region of Warsaw to his brother Casimir, retaining the regions of Czersk, Liw and Rawa.
It was established in 1353 together with the adjacent Augustinians cloister and a hospital of the Holy Spirit intra muros by Siemowit III duke of Masovia and his wife Eufemia.