When I needed to do a bit of medieval research, I went to the children's section in the local bookshop. I bought one of those books that shows everything as a cut-away diagram, you know the sort of thing – a castle with one wall removed so you can see inside. I found this really useful. It was full of helpful little snippets of info, as well as being very visual.
Yes, but it's always the same ##### castle with 4-towered square keep and inner and outer wards, isn't it? Strangely, there aren't a lot of those about, and when they do exist, they usually have an odd shape due to the builders exploiting the ground.
There's also a big distinction between castles thrown up to a grand plan in one semi-continous building campaign, e.g. Carnarvon, and more intimate castles bodged together organically over several generations, e.g. Direlton.