In the 11th century, the Low Countries were not much urbanised yet, and their armies were as feudal as everybody else's. There were some cities already, but they are rarely said to have played any significant part in the wars of that time... it's all dukes and counts leading their knights.

Later on, the cities of Flanders are very significant and everybody knows the battle of the Golden Spurs. But even that was only a small part of the low countries... most of it remained poor and rural until the early modern age.

So, I doubt there was much good quality urban militia to be found in the low countries in the 11th century.