I am indebted to the kind folks of the SCA Heraldry Chat group in Facebook, to which I posted the image.

Oddly enough, this is the back side of the flag. Oddly because charges with an orientation usually face the right side of the coat of arms, as in the picture.

SCA name Marcel van Vlaanderen writes, "The shield with the key is of the German town of Linz am Rhein. http://www.ngw.nl/heraldrywiki/index.php?title=Linz_am_Rhein" That says that it can be seen in the first seal (with a larger cross section and smaller key section), but doesn't provide a citation for that. The coat can be seen at the city's Web site.

He also notes that the other is Remagen, across the river Rhein; http://www.ngw.nl/heraldrywiki/index.php?title=Remagen . The city's Web site has a page about its history, with a picture and text at the bottom, and what may be that earliest seal at the top.

Oh, and the castle-y thing on top is a mural crown being used as a crest (a design above a shield or shields), often done in municipal heraldry.

You may have better luck in /r/vexillology/.