On the top of Wawel Hill lies the Gothic Wawel Castle. It is built up of several structures around a central courtyard. The structures have been added and expanded in different steps by different people over time. Some of the most ancient ruins date back to the 11th century. The castle has been the home of several Royal families. The hill lies on the bank of the Vistula river and is a dominating feature of the landscape. For this reason it has been a valuable military stronghold, but also a center of European trade. Apart from the private royal rooms and treasury there is also a large cathedral with royal tombs. There is also a cave "Dragon's Den" which is supposed to have been the home of the legendary dragon of Wawel. Today the castle serves as a museum, and is part of Cracow's Historic Centre, which is listed as an UNESCO World Heritage Site.