It really depends on the size and location you want the castle to be. Don't be afraid to use google and search up real castles and even chateaus to get a good feel for the areas they were built in. A plateau surrounded by natural defenses (mountains, hills, rivers, etc.) is always a good one as well, but unless you want to do a lot of terraforming they can be restrictive on the size of your castle. The first few castles I ever built on minecraft were originally strongholds that I made generate above ground and then transformed them into castles by adding more levels etc, and I actually found that to be a really good way to get used to building them and it made designing them from scratch a lot easier after that.

I think it should be surrounded by hills to make it have more of a defensive feel.
I also think it should have stone and stone brick for walls and dark wood planks for floors.

That's all I got.