Vaxholm Fortress was once an incredibly important part of
Stockholm,
Sweden's defenses. However, by the 19th century if had become so militarily useless that, according to our article on it, "the great Prussian Field Marshal Von Moltke was only ever seen to smile twice. Once when they told him his mother-in-law was dead and again when he saw Vaxholm Fortress."
(Photo by ArildV)