Nah, can't agree at all with this. Hyrule Castle isn't close to a feature player in most of the Zelda games. After Ocarina only 2 Zelda games have featured Hyrule Castle as a centerpoint. Meanwhile, Majora's Mask, Wind Waker, Phantom Hourglass, Spirit Tracks, and Skyward Sword all more or less ignored it save for a few moments here and there, and Breath of the Wild looks to do the same. I'd argue it's far from the most important aspect of the series. That would be Link, Zelda, and Ganon. Put them in there, and build an adventure fantasy world around them, and you have more than enough