I was one of the people who questioned Arundel on your original list, simply because there are so many big hitters (cathedrals, stone circles, historic ships) around your Winchester base that it seemed to stick out as an outlier.
But it then became clear that you’re particularly interested in castles, and as Winchester isn’t a castle-y area I understand your desire to go to Arundel.
Arundel is an old mediaeval castle much rebuilt as a manor house . It’s very pretty, as is the town. If you take the South Downs route (A272) it’s very pretty and there are some nice antiquing towns nearby (Midhurst, Petworth) if that’s your sort of thing. I feel like I’m damning it with faint praise but I’m from that part of the world so I may just be blasé. It seems other visitors are more impressed than me, and if it calls to you, you should go.
Kenilworth is a big chunk of ruined castle, very evocative, with lots of walls you can walk on. A genuine sense of history, but you need to understand the relationships between the various ruined bits to get a sense of the whole castle. One tower still has rooms, and you’ll get a great sense of Elizabethan era from that and the restored garden.I wouldn’t go all the way to Kenilworth from Winchester unless combining it with something - Warwick Castle or Stratford Upon Avon are the obvious choices.
I forget from your original post if you have children/teenagers. If you do, their favourite castle would probably be Warwick, near Kenilworth, which - while a bit Disneyfield - has that castle “wow!” factor as well as lots of historical recreation stuff.
The best castle (for me) in England is Dover Castle, an easy day trip from London.
Oh! Extra thought: are you interested in Roman stuff? You could double Arundel with popping down to Chichester and then seeing Fishbourne Roman Palace - extremely well-preserves mosaics in a well-designed 60s building that sits above the palace site and gives you a great sense of the Roman Palace.
