Cidadela Petersberg

This only largely preserved baroque city fortress of Central Europe, on the site of a former Benedictine monastery, is due to the rule of the Kurmainz family in Erfurt and a reflection of European fortress architecture. For centuries, it has shaped the cityscape of Thuringia's state capital. Star-shaped it was considered the most modern plant of the time and as impregnable. The mysterious Horchgänge the fortress can be visited with a guide. The small military history museum at the entrance shows the former living conditions within the plant.
Source: erfurt-tourismus.de/sehens-wissenswertes/sehenswertes/zitadelle-petersberg

The foundation stone of the Petersberg Citadel was laid on June 1, 1665, and the first of three construction phases (1665–1702) began. Among other things, the eight bastion walls in the neo-Italian style and the commandant's house with the baroque main gate Peterstor (Wikipedia) were created.

The Zidatelle Petersberg is currently a huge construction site in preparation for BUGA 2021 and is still well worth seeing. This well-preserved baroque city fortress is a reflection of European fortress architecture. From the cathedral square it is only a few steps to the fortress, which offers a breathtaking view over Erfurt; you can even see the Buchenwald bell tower from here.
There is a vineyard on the southern slope of the Petersberg citadel. Definitely plan a lot of time, is absolutely worth seeing