Hello!

Flanders was an interesting case in the medieval period as it formed an economic and cultural antipode, of sorts, to the cities of Northern Italy (which also developed a culture focused around economic prosperity and cultural dominance of the Italian peninsula). Flanders, as part of the Kingdom of France (de jure), developed a degree of autonomy not seen in other parts of the kingdom, and part of that was because of their culture of urbanisation and economic development.

Flanders developed its economy based on wool and cloth. They would weave wool imports from neighbouring lands into cloth, which would they be used for domestic purposes or exports, which in turn brought a great deal of money and attracted merchants to the main cities of the region - Brugge, Gent, and Ieper. This influx of wealth allowed for a steady degree of urban development compared to many other parts of Europe - and most similar to urban development patterns along river systems in other parts of the Holy Roman Empire. This development allowed Flanders to become the centre-point of trade in Northern Europe, with merchants from the British Isles, France, Germany, and Scandinavia all converging in one place and allowing for sustained growth that was fostered primarily by the cloth trade.

Flanders' primary economic partner in this period was sustained by England, as the wool trade became profitable across the English Channel as well. English wool became known as the "most prized in Medieval Europe", and this consistent trade allowed Flanders to consistently create cloth which was then sold in the rest of Europe for it. Even after Flanders became part of Burgundy, the cloth trade continued, although it was in decline from the 14th century onward as production shifted across the Channel to England (resultant of taxation on England's part), with the Hundred Years' War causing a decline in trade (and depriving the Flemish of their preferred wool, instead causing a shift towards merino sheep from Spain [and marking the first appearance of Spanish merchants]).

I hope this could be of some help to you!

Sources:

John H. Munro, 'Medieval Woollens: Textiles, Textile Technology and Industrial Organisation, c. 800–1500', in The Cambridge History of Western Textiles, Volume 1, ed. by D. T. Jenkins (Cambridge: Cambridge University Press, 2003), pp. 181–227 (at 186–89).

Discovering Belgium, "The Rise and Fall of the Medieval Flemish Cloth Industry", April 2018. https://www.discoveringbelgium.com/medieval-flemish-cloth-industry/