Final answer:
During World War II, the French retreated to the fortress-city of Dunkirk where they were evacuated by a flotilla of vessels back to England.
Step-by-step explanation:
The fortress-city that the French retreated to was Dunkirk during World War II. In late May 1940, over 300,000 British and French soldiers were trapped on the coast of the English Channel near Dunkirk. The British Royal Air Force held off the German Luftwaffe while a flotilla of navy and fishing vessels managed to evacuate the soldiers back to England, making the retreat a success for the Allies.