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I cannot find a function for them; what is the point? Just marking each player's starting half-domino?
The main point of the castles is to confirm your player colour. You could use your second king meeple to do the same thing, except in a 2-player game where you use both meeples.
Right now, they show your color.
And yes they have no real purpose in the game. But a kingdom needs a castle, so I think it is a nice addition.