in the time of Julius Caesar the area north of the Lake of constance was still celtic. The germanic Suebic tribes came arround the year anno domini 100 - 250 - and not all Celts left the area of southern Germany. they mixed with the suebic tribes and were named Alamannen = all men. Between Stuttgart and Munic many geographic names, all major rivers have celtic names. Up to today there are also celtic words in the local suebian/alemann dialects