Petersberg Citadel is the military counterweight to Erfurt's cathedral hill: a broad Baroque fortress whose bastions still dominate the western edge of the old town. From the ramparts you can look across the cathedral, Severikirche, the tiled roofs of the centre, and the flat Thuringian plain beyond.
The site rewards more than a quick photo stop. Guided routes lead into the underground passages, while the open grounds make a good slow walk after the cathedral square. Its scale also gives Erfurt a different historical register, showing the city as a defended strategic prize rather than only a medieval trading town.