Frankfurt am Main under Siege in 1552 (c. 1555) Forces under Elector Moritz of Saxony besieged the city of Frankfurt am Main during the revolt against Emperor Charles V in 1552. Though Protestant, Frankfurt sided with the emperor and withstood the three-week siege until it was freed by Imperial troops. Drawing by Johannes Glauburg and Johannes Völker (after Conrad Faber von Kreuznach), c. 1555.