From 1050 to 1571, the city became very important because it was on key trade routes. Many of the parliaments of the Holy Roman Empire (called the "Reichstage") met at Nuremberg Castle. The Diets of Nuremberg were an important part of the administrative structure of the empire. In 1219, Nuremberg became an Imperial Free City under Emperor Frederick II. This meant it was ruled by the Emperor, not one of the local lords or princes. It also meant that Nuremberg had a seat in the Imperial Parliament.