The roof thing intrigues me. First I though there "had to be" a wooden roof, but then I though of the interior stone (apparantly) floors of a palace in italy I once saw. But a flat roof would rise a question of how to shelter the building from the rain.

If you feel the roof is a bit big and empty, I guess castle owners would use this space for extra buildings.

Reminds me of the times I played role-playing-games. Drawing castle plans was great fun.