Seems more like the rule is:

> 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.