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