There is evidence of Celtic and Germanic settlements in the city dating from the 1st century bce, as well as Roman remains from the 1st and 2nd centuries ce. The name Frankfurt (“Ford [Passage or Crossing] of the Franks”) probably arose about 500 ce, when the Franks drove the Alemanni south, but the first written mention of Franconofurt stems from Charlemagne’s personal biographer, Einhard, in the late 8th century. The Pfalz (imperial castle) served as an important royal residence of the East Frankish Carolingians from the 9th century through later medieval times. In the 12th century the Hohenstaufen dynasty erected a new castle in Frankfurt and walled the town. The Hohenstaufen ruler Frederick I (Frederick Barbarossa) was elected king there in 1152, and in 1356 the Golden Bull of Emperor Charles IV (the constitution of the Holy Roman Empire) designated Frankfurt as the permanent site for the election of the German kings.