Yeah well, the feudal lords weren't all that big shots either back then (already given their sheer numbers and fractitiousness). And just about the first thing burghers tended to do if and when they got off the feudal yoke was building as strong a wall around their town as they could afford, and setting up a militia to keep it safe. The latter would have been fairly easy actually, as urban centres in the feudal system were due troops as much as anyone...

Given that the Low Countries region sits at the mouth of around the biggest navigable rivers in Western Europe, and is generally well-placed for trade, one suspects the local burghers could build some fairly hefty walls indeed...

Anyway, that they had enough quality infantry that they could export it as mercenaries suggests a fair bit about the region too.