Lviv is located at geographical, on the verge of the Roztocze Upland, approximately seventy kilometres from the Polish border. Lviv's altitude averages 289 metres above sea level although there are many hills located within the confines of the city. The city's highest point is Vysokyy Zamok (the High Castle), a hill of 409 metres above the sea level. The old city, surrounded by walls, was located at the foothills of the High Castle and the banks of the river Poltva. In the 13th century the river was full of water, and used for commerce and transporation of goods. In the early 20th century, as the river became polluted, it was covered where it flows through the city. The central street of Lviv, Independendance Avenue (Prospect Svobody) is right above the river, as well as the famous Opera House.