As far as I can see these prices are only a small saving over what you'd pay a short while in advance - e.g. for tomorrow, which allows better checking of the weather if you have a long journey and wouldn't find their "rainy day" offer much use.
Park over the road at St Nicholas for £4.50 all day, rather than £10 at the castle.
I would be taking my retired old man who has never been in a castle lol
It's all pretty good but an extra fiver for the dungeon does seem a bit of a rip off, especially at this time of the year when there's much less to see outside. Personally, I'd say the interiors are really better for older kids who are interested in the fascinating history of this place; without the outdoor activities there's less to interest younger kids.
Is it worth paying to see the Dungeon or just the castle?