Hi Piancol,
Welcome to the forum. Hope you enjoy the discussions and all the stuff you can find here.
Regarding your question, I would just say that if your opponents allowed to you complete such a juicy city plus three castles, they had it coming. The points galore is perfectly valid after scoring 4 times the city, once for the city itself and three more times for the castles.
Other players could have tried to block the city or complete some roads to trigger castle scoring with less points. This is why it is important to keep an eye on other player's castles and try to disable them even by sacrificing some points "for a greater good."
As pointed out by Willem, the only issue is that the city scored 2 extra points and therefore each castle as well. I agree the issue may come from the tile marked with a black square that could be counted twice in the heat of the moment (see image attached).
In any case is a master move, so congratulations for completing the city and the three neighboring castles.
Cheers!