City (pop., 2000 est.: 200,800), southwestern Belgium.
Following the Treaty of the Pyrenees (1659), in which Spain was ceded French territory, Spain in 1666 decreed that a new fortress, named for Charles II of Spain, be built there, at the site of a medieval village. It was strategically important in the 17th–19th centuries and was held variously by France, Spain, Austria, and Holland. Though the fortress was dismantled in the late 19th century, the area retained its strategic importance; it was the scene of one of the first battles of World War I.