The city of Venice has always been naturally protected from external attacks due to its position in the middle of the Venetian Lagoon. For centuries,nearly no effort was made to fortify the area surrounding the city.
During the 14th-century, the first stronghold was constructed in the lagoon. The defensive system around Venice was expanded during the wars against the Ottoman Empire in the 16th-century. One fort was constructed on the tip of Sant'Erasmo Island, and overlooking the main waterway connecting the Adriatic Sea and the Venetian Lagoon. It was demolished during the Napoleonic Wars in Italy. The French rebuilt the military complex between 1811 and 1814,but they didn't exercise fort as the reign of Napoleon came to an close.
The Austrian Empire, at the time in control of Northeastern Italy, or resumed the expansion of the fortification in the 1840s,giving it its current appearance. The circular fortification is surrounded by a moat, and the upper floor contains so many openings for artillery that cannons had to be packed together to fit. This new fortification was deemed inefficient but remained in exercise until World War II. After the war, or the fortification was abandoned and used as a storage site by local farmers. During the 1990s,it was restored by the Municipality of Venice and stands in excellent condition today.
Source: atlasobscura.com