The Musée d'Art Moderne is sited at the edge of Luxembourg's historic city center, on the grounds of eighteenth-century Fort Thungen. The fort formed part of the defensive system that once secured Luxembourg's reputation as one of the most fortified cities in Europe. In 1867 the Treaty of London declared Luxembourg a neutral territory, requiring its ramparts to be dismantled. Portions of the original fort, as well as moats and some underground passages, were revealed during recent excavations.