This is the symbol of the city, made of azure with a crenellated silver tower, and also an escutcheon covered in gold and azure. On it, you can also see the laurels which refer to Hercules.
The oldest representation known of these armorial bearings can be found on the token of the mayor Bourgeois, in 1555. We can however consider that they date back long before this time. Indeed, the presence of the old Burgundian shield alone, either expresses a ducal concession, or signifies that the castle of Semur was a ducal ownership during the Capetian house of Burgundy era (before 1361).
From « Armorial des villes et bourgs chefs-lieux de cantons de la Côte d’Or » Jean Marillier, les éditions du Bien Public