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Middle Ages

Lambert I of Leuven, Count of Leuven, gained the County of Brussels around 1000, by marrying Charles' daughter. Because of its location on the banks of the Senne, on an important trade route between the Flemish cities of Bruges and Ghent, and Cologne in the Kingdom of Germany, Brussels became a commercial centre specialised in the textile trade. The town grew quite rapidly and extended towards the upper town (Treurenberg, Coudenberg and Sablon/Zavel areas), where there was a smaller risk of floods. As it grew to a population of around 30,000, the surrounding marshes were drained to allow for further expansion. In 1183, the Counts of Leuven became Dukes of Brabant. Brabant, unlike the county of Flanders, was not fief of the king of France but was incorporated into the Holy Roman Empire.

In the early 13th century, the first walls of Brussels were built, and after this, the city grew significantly. Around this time, work began on what is now the Cathedral of St. Michael and St. Gudula (1225), replacing an older Romanesque church. To let the city expand, a second set of walls was erected between 1356 and 1383. Traces of these walls can still be seen, although the Small Ring, a series of boulevards bounding the historical city centre, follows their former course.