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At first it was part of another Polish fiefdom, the Duchy of Masovia, as in 1388 the king of Poland, Władysław Jagiełło, granted Belz to Siemowit IV, Duke of Masovia, for his recognition of Masovia as a fiefdom of Poland and as a dowry for Siemowit's marriage with Jagiełło's sister, Alexandra.
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.
Duke Siemowit, being ambitious, was very active in politics of the period and thus needed cash to finance his policies so several times he mortgaged some of his domains to the Teutonic Order, including Wizna, Zawkrze and Płońsk.
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.