Castles are a nice addition, but I feel like they could be improved
First, since castles are permanent, when you are naming a castle there should be a 'cancel' option right next to the confirm which allows you to go back and place it somewhere else. I've accidentally placed castles right on top of another when I was just trying to see the name. Maybe also have all castle names be visible when you are placing another one.
Second, the 'Build Castle' card should be renamed to 'Build or Upgrade a Castle', which would allow you to click on an already existing castle to upgrade it, making the surrounding area even harder to conquer and perhaps extending the radius a little. This would make the card more beneficial for smaller nations or ones that are already covered with castles and would also allow you to designate capitols or important locations.
Lastly, since castles tend to build up late game I think every time a tile with a castle is conquered there should be a chance, maybe 25%, of that castle being destroyed. Upgraded castles would be harder to destroy this way.