Kuusisto Castle was a medieval episcopal castle built in the beginning of the 14th century. It was the main residence of Finnish Catholic bishops until 1522, when the last bishop Arvid Kurki drowned when fleeing from Danish soldiers.
Kuusisto heyday was in the in the beginning of the 15th century, when bishop Maunu Tawast spent lot of time and money to enlarge the castle. The castle was ordered to be demolished during the Protestant Reformation in 1528 by the king Gustav I of Sweden. Stones of the castle were later used in renovations of Turku and Kastelholma castles.
Excavation and reconstruction work on the remaining ruins began in 1891.
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.