While the builder of the Aarberg Castle is unknown, the city itself was founded between 1220 and 1225 by count Ulrich III of Neuchâtel. The count had recently acquired the rulership over this region and needed a central location from which to rule. The island and the key bridge was a natural location for a town. The castle may have been built around the time of the founding of the city.
The city was besieged in 1339, 1382 and 1386 but not taken. In 1358 the Graf (or Count) Peter von Aarberg was in financial difficulties and began looking for someone to buy the city. After years of unsuccessful attempts, in 1377-79 he was able to sell the city and his rights as ruler to Bern. The Bernese bailiff took up residence in Aarberg Castle in 1379. In 1380, the old castle roof was replaced with a new shingle roof. In 1419 and again in 1477, the town and castle of Aarberg were nearly destroyed in a fire. Both the town and castle were rebuilt following the destructive fires.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.