The main defensive structure at the entrance to the city (today’s Piazza Antonio da Sangallo), its walls, by the end of the 18th century, had been connected to the Porta Pia Gate with further fortifications. The fortress is named after Antonio da Sangallo the Younger, who built it for Pope Clement VII (1532 to 1535).

The five bastions of its star shape layout, each facing in a different direction, have evocative names: Gregoriano, del Cavaliere a basso, della Campana, della Beltresca and del Giardino. A fortified approach, plus the city's defensive wall, designed by Francesco Paciotto provided strategic reinforcement.

Inside the Cittadella Park there are pathways for the sight impaired. From the highest point of the green area there is a 360° view of the entire city stretching from the port to Mount Conero.