Towering in the Bielsko-Biała city centre, the Castle is the oldest and largest construction of historical importance, erected in the old town of Bielsko. A legend says that in its place there used to be a settlement of robbers who attacked travelling merchants. The Opole Prince, Casimir (1229/30) of the Piasts is said to have conquered that fortalice, wiped out the robbers and had the hunting palace erected in that place, which over the years grew into a magnificent castle around which the city of Bielsko developed. The oldest part of the Castle dates back to 14th century. Over the next centuries the Castle gradually developed and transformed. It is a city castle in its nature, incorporated into the system of Bielsko fortifications from the beginning, at the same time providing their strongest section. Over the centuries it performed the function of a Silesian border-stronghold, first guarding the borders of Cieszyn and Oświęcim district duchies and then in the second half of the 15th century it protected the Czech and Polish state border and from 1526 - the Austrian-Polish border. Starting from the close of the 16th century, its defensive role was declining and the Castle gradually transformed into a nobleman’s mansion. The present appearance of the castle dates back to the last, thorough reconstruction undertaken in the second half of the 19th century, which entirely wiped out its previous characteristics of style. During the years 1899-1973, in place of brick breast wall presently seen on the east part of the Castle, there used to be a parade of bazaars, constituting an attractive architectural foundation for the body of the Castle. The bazaars were pulled down in connection with widening of Zamkowa Street. The Castle erected by Piasts ruling over the Cieszyn Dutchy was one of their residences for over two centuries. From 1572 it was the administrative and commercial centre of the independent class-based Bielsko state, governed by representatives of nobleman’s families of the Promnitzes, Schaffgotsches, Sunneghs, Solmses and Haugwitzes. In 1752, the position of that state was raised to the position of duchies which went into the rule of the Sułkowskis family. The Bielsko Dutchy existed until 1849, when Austria introduced modern administrative division, thus doing away with old feudal structures and was incorporated into Bielsko District Starosty. The Castle itself and numerous estates in the vicinity of the city remained in the possession of the Sułkowskis until 1945. After World War II the Castle was taken over by the Polish State as the property left by the Germans and was facilitated as the seat of many cultural institutions. Since 1983 the Castle sole usufructary has been the national Museum in Bielsko-Biała, subordinated to Silesian local government in Katowice.
Source: https://en.wikipedia.org/wiki/Bielsko-Bia%C5%82a_Museum_and_Castle