The Royal Coin Cabinet is a museum dedicated to the history of money. The museum includes exhibitions of coins, banknotes (the first in the world was issued in 1661 by Stockholms Banco), treasure hoards and piggy banks. Particularly notable are the oldest Swedish coin, a copper plate coin dating from Queen Christina's reign in 1644 that is thought at 19.7 kg to be the heaviest in the world, some of the Lohe treasure found in 1937 on Gamla Stan, Weimar Republic inflation currency and some Nobel prize medals.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.