In the 1540s, the fortifications of Messina were being modernized due to fears of the expanding Ottoman Empire. Forte Gonzaga was built on the hill of Montepiselli, outside the city walls. It was able to defend the mountainous landward approach to the city, and it also overlooked the Strait of Messina. The fort was designed by Antonio Ferramolino, a military engineer from Bergamo. He was assisted by Francesco Maurolico, a native of Messina. It was named after the Viceroy of Sicily Don Ferrante Gonzaga, and was completed in 1545.
The Ottoman threat was reduced after the Catholic victory in the Battle of Lepanto in 1571, and the fort's importance began to decline. It saw use during the 1674–78 uprising against Spanish rule. Spain eventually lost Sicily in 1713, but invaded the island five years later during the War of the Quadruple Alliance. During the invasion, the fort did not offer much resistance and was captured by the Spanish general Luca Spinola.
During the Sicilian revolution of 1848, the fort was captured by rebels, who used it to bombard the Real Cittadella which was still in Bourbon hands.
Forte Gonzaga saw use in World War II when it was used by German and Italian forces prior to the Allied invasion, and it was subsequently used by American forward observers to direct artillery fire during the invasion of Italy. It remained a military establishment until 1973, when the Italian Army handed it to the municipality of Messina. There are plans to restore the fort and turn it into a museum and conference centre.References:
The Petersberg Citadel is one of the largest extant early-modern citadels in Europe and covers the whole north-western part of the Erfurt city centre. It was built after 1665 on Petersberg hill and was in military use until 1963. It dates from a time when Erfurt was ruled by the Electors of Mainz and is a unique example of the European style of fortress construction. Beneath the citadel is an underground maze of passageways that can be visited on guided tours organised by Erfurt Tourist Office.
The citadel was originally built on the site of a medieval Benedictine Monastery and the earliest parts of the complex date from the 12th century. Erfurt has also been ruled by Sweden, Prussia, Napoleon, the German Empire, the Nazis, and post-World War II Soviet occupying forces, and it was part of the German Democratic Republic (East Germany). All of these regimes used Petersberg Citadel and had an influence on its development. The baroque fortress was in military use until 1963. Since German reunification in 1990, the citadel has undergone significant restoration and it is now open to the public as a historic site.