I am still unsure how I feel about the direction this show takes. I did like it in general even if it is not the continuation of the 80s I hoped for after the first news about the show.
I get your point, a Masters of the Universe without He-Man would be strange to a point, where I would have problems to recognize it as MOTU at all. The only way I could see such a plot to work would be, that the show will take a full apocalyptic direction to a world without its most powerful hero and the remaining Masters fighting desperately in some kind of a Guerilla-war against the Evil Warriors (a plot I would have preferred when the Horde enters the picture, but I think it could work with a Skele-god).
But I really hope they let Adam/He-Man return once more from the dead, sounds like a stupid idea to do MOTU without him and they already killed enough heroes just for the sake of killing them off.
I'm not sure if Crystal Castle ever got a special reason of existance in the show other than being the seat of Light Hope. I don't remember it's significance in the Minicomics. In germany we had some audiobooks about She-Ra, but they were... well... far away from canon and often refered to the princesses of power as fairies (f.ex. Catra was called "the Cat-Fairy"), but there the Crystal Castle was She-Ras seat of power and residence, the most beautiful palace in all of Etheria and Catra and her secret master, the wizard of the volcano, always tried to get the castle for themselves. It was told to be of significance for She-Ra and a source of power, but that was never totally explained.
That's all I remember about it. Hope it helped even if it was not that much about Crystal Castle in the Filmation-Show.