It is the unpaved pedestrian square located between the Castello Sforzesco and the Parco Sempione. It owes its name to a cannon placed in the middle of the square and removed around the 1950s.
Someone tries to return to the ancient splendour by smoking a few cannoni (joints), but that’s another story.
The photo is by Federico Patellani (1911-1977), from the site of the Cultural Heritage of the Lombardy Region. “Referendum 1946. Milan. Piazza del Cannone. Monarchic manifestation. Crowd sheltered under umbrellas with stickers of the monarchic flag – in the background the inner perimeter wall of Castello Sforzesco”.
Translated by @silaskin