It has something to do with the combination of CaptureWorld and RenderWorld. This is where I read about the castle demo: http://www.blitzbasic.com/b3ddocs/command.php?name=RenderWorld
Here is another thread about it:
http://www.blitzbasic.com/Community/posts.php?topic=37621
Here is another thread about it:
http://www.blitzbasic.com/Community/posts.php?topic=37621