Regensburg is a city in eastern Bavaria, at the confluence of the Danube, Naab and Regen rivers, Danube's northernmost point. It is the capital of the Upper Palatinate subregion of the state. With more than 150,000 inhabitants, Regensburg is the fourth-largest city in the State of Bavaria after Munich, Nuremberg and Augsburg and the 8th largest of all cities on the Danube river. From its foundation as an imperial Roman river fort, the city has been the political, economic and cultural centre of the surrounding region. Later, under the rule of the Holy Roman Empire, it housed the Perpetual Diet of Regensburg.