The settlement of Ostrzeszów predates the advent of Christianity in Poland in 966. In antiquity, the Amber Road ran through the area. Ostrzeszów acquired town privileges sometime between 1261 and 1283, when it first appears in historical records. In the 14th century, Polish King Casimir III the Great erected a castle, defensive town walls and the Gothic church of the Assumption of Mary. The town's coat of arms was probably granted by Casimir's successor Louis the Hungarian. It contains the head of the crowned white eagle from the coat of arms of Poland. Ostrzeszów was a Polish royal town and county seat within the Sieradz Voivodeship in the Greater Poland Province of the Polish Crown. During the 16th to 18th centuries it was the seat of district courts. Ostrzeszów prospered in the early modern era, multiple craft guilds were founded and trade flourished until the Swedes destroyed the castle and the town in 1656 during the Swedish invasion of Poland ("Deluge"). The town declined as a result.