6th century onward during the migration period. The Ślężans, a West Slavic tribe, settled on the Oder river and erected a fortified gord on Ostrów Tumski. Wrocław originated at the intersection of two trade routes, the Via Regia and the Amber Road. Archeological research conducted in the city indicates that it was founded around 940. In 985, Duke Mieszko I of Poland conquered Silesia, and constructed new fortifcations on Ostrów. The town was mentioned by Thietmar explicitly in the year 1000 AD in connection with its promotion to an episcopal see during the Congress of Gniezno. Middle Ages During Wrocław's early history, control over it changed hands between the Duchy of Bohemia (1038–1054), the Duchy of Poland and the Kingdom of Poland (985–1038 and 1054–1320). Following the fragmentation of the Kingdom of Poland, the Piast dynasty ruled the Duchy of Silesia. One of the most important events during this period was the foundation of the Diocese of Wrocław by the Polish Duke and from 1025, King Bolesław the Brave in 1000. Along with the Bishoprics of Kraków and Kołobrzeg, Wrocław was placed under the Archbishopric of Gniezno in Greater Poland, founded by Pope Sylvester II through the intercession of Polish Duke Bolesław I the Brave and Emperor Otto III in 1000, during the Gniezno Congress. In the years 1034–1038 the city was affected by the pagan reaction in Poland. The city became a commercial centre and expanded to Wyspa Piasek (Sand Island), and then onto the left bank of the River Oder. Around 1000, the town had about 1,000 inhabitants. In 1109 during the Polish-German war, Prince Bolesław III Wrymouth defeated the King of Germany Henry V at the Battle of Hundsfeld, stopping the German advance into Poland. The medieval chronicle, Gesta principum Polonorum (1112–1116) by Gallus Anonymus, named Wrocław, along with Kraków and Sandomierz, as one of three capitals of the Polish Kingdom. Also, the Tabula Rogeriana, a book written by the Arab geographer Muhammad al-Idrisi in 1154, describes Wrocław as one of the Polish cities, alongside Kraków, Gniezno, Sieradz, Łęczyca and Santok. By 1139, a settlement belonging to Governor Piotr Włostowic (also known as Piotr Włast Dunin) was built, and another on the left bank of the River Oder, near the present site of the university. While the city was largely Polish, it also had communities of Bohemians (Czechs), Germans, Walloons and Jews. In the 13th century, Wrocław