The sections of the city that date from the earliest origins of the city constitute the lower town (bas de la ville). The name derives from the area's geographic location in the Senne River valley below the Coudenberg and Treurenberg hills, on, around, and beyond whose heights lies the upper town.
Long prone to flooding when the Senne overflowed its banks, the area lost favor with the ruling elites, who followed the counts of Leu-ven to the upper town when the latter built their fortress-residence there in the 11th century.
The lower town comprises the medieval heart of Brussels, which grew around the Grand' Place and the port alongside the river. A commercial area from its beginnings, it still preserves the network of narrow, winding streets and lanes around the Grand' Place at the same time that other districts have undergone much rebuilding with modern steel and concrete structures. The Îlot Sacré district, much renovated, has attracted growing numbers of upscale residents.
Associated with trade and industry, municipal autonomy and urban prerogatives, the lower town acquired over time a pronounced working-class character. As such, it stood in marked contrast to the aristocratic upper town, where the rulers of Brabant resided.
Historical Dictionary of Brussels. Paul F. State.