According to the French Wikipedia (Drapeau de l'Alsace), the "Rot und Weiss" dates back from the German annexation after the 1870-1871 Franco-Prussian War, not from the Middle Ages. The colours (red and white) are traditional in Alsatian local and dynastical heraldry, but the flag itself seems to be from the 1870s. According to this source, it was first flew by the town of Strasbourg in 1872._"http://drapeaualsace.free.fr/nouvellepage3.htm"