Gary, I understand that the world of Aerth is a fleshing-out of what was in part undeveloped and/or implicit in your original Greyhawk campaign world. How did the counter-earth of Phaeree figure into your original D&D campaign world? Did you always envision most of the non-human critters (elves, giants, dragons, etc.) as being "away from home [i. e., Phaeree]" when on Oerth? Or was Aerth's Phaeree a later conception? In either case, it is a most intriguing way of presenting a solid ecology for a fantasy world. It allows for a magical "alternate Earth" setting, allowing it to remain humanocentric and answering the question, "How can these monsters be here without totally screwing-up the world's ecology?" [Answer: The monsters aren't native to the campaign world. They are interlopers from another world, arriving here through magical gates.]
And once again let me congratulate you on your Epic of Aerth book. It is simply packed with good stuff. It allows one (with ease and facility) to incorporate ANYTHING he reads (mythology, legendry, fantasy or horror fiction) or watches (fantasy and old Universal studios films, etc.) into the campaign world. There is a ready-made slot for everything, whether the various lost worlds of A. Merritt, Posnansky's Tiahuanaco, Chinese gods, Cthulhu, or what-have-you. I truly believe that the Epic of Aerth book is one of the classics of FRPG books, ranking up there with your AD&D Dungeon Masters Guide.
Also, regarding your Castle Zagyg products: How high a level of characters can reasonably adventure in the depths of the castle's dungeons?