This weekend I went to see an exhibition about a “궁”, and the exhibition was held at a “성”. That led me to think about how these two words differ.
In English the words “palace” and “castle” are both used to describe a place where royalty lives, though castles tend to have more towers and sometimes also a moat (think Windsor Castle vs. Buckingham Palace or Kensington Palace style-wise).
That being said, though, you wouldn’t call Windsor Castle or Buckingham Palace a 궁 even if they don’t fit under exactly the same label in English.
The difference between 궁 and 성 is found in the architecture on a larger scale than the difference you find between European style palaces and castles:
Those of you who have seen a Korean palace in real life or in dramas will have noticed that rather than a single building, the palace grounds are more like a little village containing a many separate houses. No particular house is considered “the 궁”, they are all part of the 궁.
In contrast, a castle or a European-style palace consists of one building and it has multiple storeys. There may of course be other buildings near/associated with the castle (for servants’ quarters or the like), but they are not considered part of the castle itself.
That means that when speaking of Korean royalty, using the word 성 would be misleading, and when speaking of for instance Buckingham Palace, the word 궁 would not be accurate either.