The Mouse Tower (Mäuseturm) is a stone tower on a small island in the Rhine. The Romans were the first to build a structure on this site. It later became part of Franconia, and it fell and had to be rebuilt many times. Hatto II, the Archbishop of Mainz, restored the tower in 968. The story of how it came to be called the 'Mouse Tower' comes from a folk tale (Hatto was being eaten alive by mice in a tower). In 1298 the structure became an official customs collection tower. It was destroyed by a French army in 1689, then rebuilt in 1855 as a Prussian signal tower.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.