I'd assume that wealthy merchants were able to hire guards, especially in areas where the aristocracy wasn't particularly powerful, i.e. Hanseatic League member states.
But I'd like to see peasant insurrections. Rebellions so powerful, that neighbouring kingdoms would have to dispatch armies to aid in quelling them, least the flames of revolution spread into their own domains. Sure they'd die down eventually, but the order of things would be upset, many nobles lying dead, estates broken up, castles ruined. Up-in-coming nobles would seize the opportunity to cast down old families that had grown weak, and install themselves as the new leaders.
