The castle (castel in Italian) is super ancient and is more like the fortress rather than castle. You will just go up and stroll outside looking over the walls to admire beautiful views of Mount Vesuvius, the gulf of Naples and city line. Honestly, there is not much to do and see there other than that, BUT it’s free and makes a lovely stop after a walk on promenade of Naples. We visited it like five years ago and it was super crowded, but this month we came back to bring my aunt who was visiting us and thanks to the required reservation needed online, there were only a few people and the visit was better this way. It felt like a relaxing time and we took some nice photos there. Highly recommend as a free cultural thing but don’t build high expectations.