I'm not quite sure what countries I'm going to be traveling through just yet, so general recommendations work for now. What are the castles and palaces that you really have to see? Preferably, they'll be somewhat reasonable to access from cities that are also great to visit. If it's something incredible enough to go completely out of your way for, I definitely am interested but I'm also trying to be realistic about time and money. Of course, if it's in a city, even better.
France is the one exception to this- I'm actually relocating to Paris for a bit, so anything that's not completely unreasonable to reach from the city is fair game, regardless of what it's by.