Citadel Petersberg is situated on a hill next to the Domplatz. This is the best preserved baroque fortress in Central Europe. It was constructed in the 17th century when Erfurt was under the rule of prince electorate of Mainz, in order to protect the city from Protestants. A huge citadel in the shape of an uneven star is surrounded by walls with total length of around 2 km. Each of eight bastions has its own name. A whole labyrinth of underground passages is hiding under the fortress, however you can only see them during a two-hour-long excursion. Territory above it is open for visits, however there are not many buildings there. Still, you should climb to the fortress at least to admire the panorama of Erfurt.