In 1543, Nice is under siege from the Sea by Barbarossa, leading a turkish fleet.
At that time, the Turkish Empire is an ally of the king of France, François (Francis) the 1st. Nice, like all the Savoy territories, belongs to Charles the 5th, King of Spain, Emperor, who also owns the Netherlands. He is at war with France. When he seizes Milano (in Italy), the king of France attacks Savoy. The Duke of Savoy flees to Nice, taking along with him the "Holy Shroud", now in Torino.
The Turks take the city but the castle, on its hill, resists. This is when the cannonball was fired against Nice.
It is at the corner of the rue Droite and the rue de la Loge, in the old town.
A few weeks later, the fleet withdraws before the reinforcements awaited by the Duke of Savoy arrive.
Other turkish cannonballs of the same battle can be seen Garibaldi square, on the blue penitents chapel.