castles in D&D

[Li Shenron]

All the castles that I have seen in D&D, have architecture like Europe. But this would not be the case if magic were around? Flying would reduce the effectiveness of walls. I realize if you played in a low magic world like conan, Newhon, or even Middle Earth, that this issue would not be so important but if your game was in the Forgotten Realms or Ebberon?

Has anyone addressed this in their world either home brew or published

Although not related to Fly we had an off-game discussion this weekend which is about a similar idea. A reclusive and secretive wizard who can cast Dimension Door could have his small keep built with no entrances, just walls and ceiling. It would be impenetrable by anyone unless they can (1) teleport or (2) breach a hole in the wall. What do you think?

Strangely, this idea stemmed from a philosophical discussion about doors between me & two friends: they were supporting the concept that the meaning of a DOOR is to keep someone either in or out, while I was saying that the meaning of a DOOR is to let someone pass. Hence I said, if you don't want someone to enter/exit, you just don't build a door