The flora and fauna that has developed over decades on parts of the citadel is now invaluable: 447 animal and plant species, including birds, bats, and wild bees, live here. Sixty-six of these species are even on the Red List, for example, the brown long-eared bat, long-horned bee, dwarf stag beetle, yew, and flaky mullein. The walls of the citadel are also among the most important moss biotopes in the country.

Another special feature is the age of the citadel walls, which means that these are habitats that have been undisturbed for centuries and, in addition to biodiversity, probably also contain ancient populations and genetic reservoirs of species.

In addition, the citadel's green spaces serve as the green lungs of the city center, which is structurally cut off from the fresh air corridors of the surrounding area. The citadel's green spaces filter the air and cool and humidify the dry, hot urban climate, extending into the residential areas. 

The high value of the citadel's green space led to the designation of the moat and part of the ramparts as a protected landscape area (GLB) in 1986 in accordance with § 29 of the Federal Nature Conservation Act. Since then, a legal ordinance has regulated the permissible actions and measures. It is now the last refuge for its inhabitants, as most of the green space outside the GLB has been cleared.