Decorative view of the city Oradey, Grosswardein , in the western part of Romania. The settlement that would become the city of Oradea is considered to have been relatively unimportant until the 11th century when St. King Ladislaus I of Hungary founded a bishopric near it, the present Roman Catholic Diocese of Oradea. The city flourished both economically and culturally during the 13th century as part of the Kingdom of Hungary. It was at this time that the Citadel of Oradea, first mentioned in 1241 during the Mongol invasion, was first built. It would be destroyed and rebuilt several times over the course of following centuries. The 14th and 15th centuries would prove to be of the most prosperous periods in the city's history up to that point. Many works of art would be added to the city. In 1474, the city was captured by the Turks after a protracted siege. Their mostly tolerant policies towards others peoples ensured that the city would become an ethnic mosaic of Romanians, Hungarians, Austrians, Slovaks, Hebrews, Ruthenians and Turks. After the Ottoman invasion of Hungary, in the 16th century, the city became a constant point of contention between the Principality of Transylvania, the Ottoman Empire and the Habsburg Monarchy. Following Michael the Brave's conquest of the Principality of Transylvania, the Ottomans sent a punitive expedition that laid siege to the city in 1598, however the siege failed. After Michael's assassination in 1601 and the Peace of Vienna of 1603, the city was permanently incorporated in the Principality of Transylvania by imperial decree. Only at the end of World War I Oradea became a part of the Kingdom of Romania under the Treaty of Trianon.