波列斯瓦夫一世
It is not clear when Zerbst was founded; however, the name "Ciervisti" mentioned as early as 949 may already refer to a fortified Slavic settlement. The chronicles by Prince-Bishop Thietmar of Merseburg recorded the first mention of a town as "Zirwisti urbs" in 1018, giving an account of the occupation by the Polish duke Bolesław I Chrobry during the German–Polish War with King Henry II in 1007. In the early 12th century the Ascanian ruler Albert the Bear had the fortress rebuilt, and the adjacent settlement was first fortified with town walls about 1250.