We did Castel Nuovo at the end of the day, when we were tired and had enough of the Italian way of organizing musea (read: cramming a lot of so-called art and other fancy objects into some rooms and provide only Italian or no information), so maybe my review isn't as objective as it should be. But I didn't think Castel Nuovo was really worth a visit.
When you enter the castle and bought your tickets, a very friendly man hands you a brochure and tells you how to get around the castle. You start your tour motivated, and enter the first room where there's a lot of art but no information. Your motivation starts to drop, but ok. You enter the next room, same problem. You're already a bit tired, so you begin to get annoyed with the Italians. Next room: still no information (but they did have money to make a 3D view of something). After that, you trudge through the rest of the museum, but only because you want to be able to say you've visited it, not because you give a [--] about the objects the Italians have collected in another one of their useless musea. It's namely hard to be interested when you have no clue about what the object you're looking at is or why it's important.
You continue to drag yourself through the museum, when you pass a balcony. Interesting, you think, fresh air to awaken your brain cells that are slowly going into sleep mode. And then you discover a very nice view over the port of Naples. After that, you hurry through the rest of the castle, pass a bunch of bones under some glass (god knows why they're lying there) and wait for the free (at least, when you've paid entrance fee) guided tour in the tower to start.
The guide is very friendly and takes you up into a tower of the castle, where he begins to open windows and gives a 10-second-explanation about which part of Naples you see through that window. After that, you get to look at stairs, so you can notice how far they extend (but no entering the stairs!). And then there was another thing he showed, but I forgot what it was. Something with a parliament from some decades ago, I think.
To summarize: I don't think the visit to the Castel Nuovo is worthwhile. The art and other objects that are stalled in the castle might be interesting, but without any explanation, I think they're quite boring. The guided tour could be nice, but the guide doesn't really tell that much. It could be a lot more interesting if he told something about the history of the castle. However, if you've got a Campania ArteCard like us, and you'd like to take some nice pictures of Naples, the port of Naples, and the Vesuvius seen from Naples, you could visit the castle.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.