In medieval Europe, as I understand it, there was always a desire for absolutism in the aristocracy that was frequently at odds with the autonomous leanings of the major cities. Cities demanded (and often got) concessions from the crown and even limited the reach of royal authority within the bounds of a city. London, if I recall correctly, could deny access by representatives of the crown, if not the king himself. In the struggle between royalty and the cities, what levers of power did they use against each other, and which cities had the most frequent and intense clashes against their lieges?