Wrzoski, Żerkowice as well as parts of Brzezie, Dobrzeń Mały and Karczów became a part of Opole, enlargening its population by about 9,500, and its area by over 5,300 ha, despite the protests of inhabitants. Historical population In the early 20th century the number of Polish and bilingual citizens of Opole, according to the official German statistics, varied from 25 to 31%. German minority Alongside German and Polish, many citizens of the city before 1945 used a strongly German-influenced Silesian dialect (sometimes called wasserpolnisch or wasserpolak). Because of this, the post-war Polish state administration after the annexation of Silesia in 1945 did not initiate a general expulsion of all former inhabitants of Opole, as was done in Lower Silesia, for instance, where the population almost exclusively spoke the German language. Because they were considered "autochthonous" (Polish), the Wasserpolak-speakers instead received the right to remain in their homeland after declaring themselves as Poles. Some German speakers took advantage of this decision, allowing them to remain in Opole, even when they considered themselves to be of German nationality. The city surroundings currently contain the largest German and Upper Silesian minorities in Poland. However, Opole itself is only 2.46% German. (See also Germans of Poland.) Main sights Opole hosts the annual National Festival of Polish Song. The city is also known for its 10th-century Church of St. Adalbert and the 14th-century Church of the Holy Cross. There is a zoo, the Ogród Zoologiczny w Opolu. Structures and buildings Piast Tower on the island, the only part that remained of the medieval Piast Castle, the local residence of the dukes of Opole Holy Trinity Church, a 14th-century Gothic Franciscan church, which contains a mausoleum of the dukes of the Opole line of the Piast dynasty a 19th-century Town Hall the Church of our Lady of Sorrows and St. Adalbert (Kościół Matki Boskiej Bolesnej i św. Wojciecha) the 14th-century Holy Cross Cathedral (Bazylika katedralna Podwyższenia Krzyża Świętego), which contains the Piast Chapel with the tomb of Jan II the Good, the last duke of Opole from the Piast dynasty The art nouveau Penny Bridge (Most Groszowy), currently named Green Bridge (Zielony Mostek) Opole Main Station, an eclectic building from early 20th century. Museums Diocesan Museum (Muzeum Diecezjalne) Opole Regional Museum (Muzeum Śląska Opolskiego) Opole Village Museum (Muzeum Wsi Opolskiej) Cemetery The Jewish Cemetery in Opole was established in 1822, and it is a peculiar pantheon