The revival of commerce, in combination with the construction of defensive walls to keep out marauders, resulted in the impressive growth of urban centers. Cities developed along the major rivers, which served as transportation arteries during this period. The towns obtain limited self-rule. Citizenship in a town was limited to males who had been living in the town for a substantial period of time. They could then apply to become members of a city council.
Despite their undeniable growth, medieval northern European cities remained relatively small. The average trading city could boast a mere 5,000 inhabitants. London, the largest city in England, had about 30,000 inhabitants in 1200 A.D.