Ostrów Tumski ("Cathedral Island", German: Dominsel) is the oldest part of the city of Wrocław in south-western Poland. It was formerly an island (ostrów in old Polish) between branches of the Oder River. Archaeological excavations have shown that the western part of Ostrów Tumski, between the Church of St. Martin and the Holy Cross, was the first to be inhabited. The first, wooden church (St. Martin), dating from the 10 th century, was surrounded by defensive walls built on the banks of the river. The island had approximately 1,500 inhabitants at that time.