The Lombardian city of Bergamo is composed of two parts, Città Alta (Upper Town), built up on the hills, and the Città Bassa (Lower Town), which is a lively financial, industrial and administrative centre of national importance.
A milestone in the history of Bergamo was its incorporation into the Venetian State in 1428, which lasted for over three centuries and a half. The two parts of city are separated, both physically and symbolically, by the powerful Venetian Walls, which were built by the Serenissima Republic of Venice in the second half of the 16th century to defend the city, which was the farthermost centre on the Mainland, close to the border with Milan's territory.
The walls never underwent any siege. That is why they remained almost intact to the present day. The system consist of 14 bastions, 2 platforms, 100 embrasures for cannons, 2 armouries, four gates, not to mention the underground structures featuring sallies, passages and tunnels.
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.