The Citadel of Besancon is a fortress located in the city of Besancon, in the eastern French region of Franche-Comte. It was built in the 17th century to protect the city from invasion and was used as a military prison until the 20th century. Today, the Citadel is a UNESCO World Heritage Site and a popular tourist attraction.

The Citadel covers an area of over 11 hectares and features a variety of buildings, including a watchtower, barracks, and a chapel. Visitors can explore the fortress and its various exhibits, which include a museum of the French Resistance, a zoo, and an aquarium. The Citadel also provides stunning views of the city of Besancon and the surrounding countryside.

In addition to its cultural and historical significance, the Citadel of Besancon is an important site for biodiversity. The fortress is home to over 250 species of plants and animals, including many rare and endangered species. The Citadel's role as a nature reserve is recognized by the French government, and it is part of the larger Besancon Vauban Citadel and Fortifications, which is a designated World Heritage Site.