Casimir III the Great
From that time, relations between Siemowit and Casimir III the Great become increasingly narrow. In 1363, Siemowit was invited to Krakow for the marriage of Elizabeth of Pomerania, granddaughter of the king of Poland, with the Emperor Charles IV. In September 1364 he participated in a conference in Krakow, which offered five crowned kings, dukes and princes extravagant festivities and tournaments. In 1369 Margaret, the daughter of Siemowit, wife of Casimir IV, Duke of Pomerania, adopted an illegitimate son of Casimir III the Great. At that time, Casimir IV of Słupsk was one of the contenders to succeed Casimir III the Great, who left only daughters.