The castrum (a diminutive of castellum) was the castle fortress built by Charles of France in 977-979. Its exact location remains unknown and historians have suggested the Île Saint-Géry or at the site of the Notre-Dame des Riches-Claires church.
The castrum included the fortified camp located on the right bank of the Senne River. Surrounded by marshes, the camp dominated both land and river routes. It probably consisted of a rectangular plot enclosed by earthen walls together with a tower, palisade, and a moat. It encompassed the area bounded approximately by rues au Beurre, du Midi, des Pierres, du Marché au Charbon, and de la Tête d'Or. The camp was soon surrounded by houses, chapels, and marketplaces. The last vestiges of the ramparts could be seen as late as 1694.
Historical Dictionary of Brussels. Paul F. State.