Where is the best place to build a castle?

Answer 1

Where it is easy defended. Near a river, some woods, or on a hill, for instance, where the castle-dwellers will have all the advantages when confronted by enemies, but also when it comes to collecting resources like wood and food for the residents to survive.

Answer 2

On the top of a hill with no trees around it.

This is correct for tactical reasons. I believe you'd want to build it on solid rock, perhaps granite. Such a location would give you both a solid foundation for your castle and rock from which to build it. Granite would mean a mountain I think, and if I were your architect, I'd try to conserve assets by building the castle into the side of the mountain itself. If your castle is for defence, this is a great idea, because you have hardly any vulnerabilities. If your castle is for comfort, build it in Las Vegas, but be aware that it will lack the drama intrinsic to a castle built into the side of a mountain.