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Each city would have its own crest, usually the heraldry of the current family or group in power. Most cities will incorporate, at least in part, the overall province symbol.

I would suggest developing a symbol for your own specific purposes rather than setting it for the entirety of the city of Calais. If it is vital that you need the Calais symbol (e.g. your character is in the ruling lineage), then something can be worked out.