Castle Howard. Went on sat and it's just stunning. More of a stately home than a castle, but more impressive than any castle I've ever seen. The scale of it is just immense. I live in Slaithwaite and it took about an hour and a bit to get there, so shouldn't be too bad from Manchester. So worth a visit. You have to go in the house if you go.
Slight hi-jack, any recommendations for Sussex way?
There's some amazing castles in the South East, but I suppose that's a bit of a drive for you! I was thinking Heaver, Leeds Castle (Kent), Bodiam Castle, Bramber, Arundel Castle...
I think one of my favourite castles I've visited is actually Corfe Castle in Dorset, it's amazing.