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.
I don't think any of these castles were built directly by kings - although I suspect their construction was either approved by a king or by someone who had delegated authority from a king. NB I can also see a large castle about ~11 km away that was a royal castle (and still has a military garrison).
I suspect that most castles are probably in other people's kingdoms.
Castles are thus more like domains where once you take hold of it, even the big powers have a hard time taking it away from you again
> Building a castle is a very good idea if you seek to entrench yourself in the power structure of the kingdom. To do this, you must be able credibly mount a defence of the castle to discourage forcible eviction without major mutual destruction (cough too big to fail).
> Don't build a wooden cottage and expect it function like a castle with a garrison under your command. Even if you slowly expand it to a stone mansion, if you don't maintain a garrison, it won't work as a castle.
Sadly, building an game on someone else's platform is more like setting up a cottage on the land. You might be able to get some farming done and survive, but if the lord fancies the grain, you're out of luck. But also good luck finding land to farm without a lord. Peasant.