The Eastphalian territory at the Harz mountain range was the hereditary lands of Henry the Fowler, the first Saxon duke to become King of the Romans in 919, and his descendants of the Ottonian dynasty. They left several Romanesque abbeys and castles, a cultural landscape that today encompasses three World Heritage Sites with the medieval town of Goslar and Quedlinburg, as well as St. Mary's Cathedral and St. Michael's Church at Hildesheim. As the Eastphalian territory bordered on the lands of the Polabian Slavs beyond the Elbe and Saale rivers, it became the starting point of the German Ostsiedlung, started by the invasions of King Henry and continued by the Saxon margraves.