The Venetians referred to them wryly as 'Viennese oranges' and they can be seen throughout Venice, often embedded into buildings.
'Viennese oranges' are the cannonballs that were fired into the city by the Austrian army during the reign of the short-lived Republic of San Marco (March, 1848 - August, 1849). Under the leadership of Daniele Manin the Venetians had rebelled against their Austrian overlords, who had ruled the city since 1815.
The Austrian troops tried to make the cannonballs more effective by making them red hot before firing, with the intention of setting fire to sections of the city. They were nicknamed 'Viennese oranges', because of the way they illuminated the night sky over Venice.