Built in 1732 as an additional defence, the fort was named after Baron Thungen, the Austrian commander. A moat surrounded the fort which could only be accessed through a 169 metre long tunnel. During the 19th Century, the Prussians extended and enlarged the fort. It was demolished in the 1870s after the Treaty of London, but the three round towers known as the Three Acorns are still standing. Foundations uncovered in 1991 are also visible.