Having now beaten the game, I spent last night's "I should be asleep already" hours combing over every inch of Hyrule Castle, inside and out. So much shit to see and find. And soooo much loot. My inventory overfloweth.
When I beat Gannon, I just snuck right up to him, entirely on the outside of the castle, so it was fun to dive in and see the rest. I had snuck into the castle and seen about 1/3 of it very early in the game, having only one Divine Beast done. It was essentially a loot run and was very cool/tense feeling of being in way over my head and really needing smart stealth tactics as well as special power and arrow usage. I wound up getting all the way to Ganon on that run, and I realize now, the game was forcing me to play through the 3 divine beast bosses before it would take me to the true forms of Ganon. So if you go in early, you are double fucked, since you have additional fights on top of likely being less prepared.
I wonder if there is a way to beat the Divine Beast bosses there and then skip out of the fight, causing them to already be beaten. I suspect it is a locked door situation once you start boss fight, but thought crossed my mind.