Kaunas Fortress (Lithuanian: Kauno tvirtovė, Russian: Кοвенская крепость) is the remains of a fortress complex in Kaunas, Lithuania. It was constructed and renovated between 1882 and 1915 to protect the Russian Empire's western borders, and was designated a "first-class" fortress in 1887. During World War I, the complex was the largest defensive structure in the entire state, occupying 65 km2 (25 sq mi).