The real solution is to force these kingdoms to build permanently open gates and roadways that connect the land, increase all around traffic and opportunity.
Only when people turn from digital vassals to digital citizens will we emerge from the middle ages we are currently in. In this sense the most important development in the online world is still ahead if us.
The reality is that if you truly want to get rid of digital castles and kings, you're essentially going to have to operate a distributed digital firehose (cynically: digital sewage pipe) that anyone can submit to with no preconditions whatsoever. For many reasons (first one that jumps to mind: spam, second reason: illegal shit, third reason: trolls) most people don't want to operate something like that, and that's before the law gets involved.
Pet projects exist of course, but pretty much zero of them are made to scale up against the idea of truly nuking kingdoms; the closest to a realization of this sort of network is something similar to TORs peer2peer, and you can consider pretty much all legal risks of running a TOR exit node for a service like this.