I have been preparing to fight calamity Ganon in Zelda breath of the wild but I do not know if it is possible to walk straight up to the castle. How do you access the castle to battle calamity ganon?
Once you get the Paraglider (or perhaps before, I don't know much about glitches or speedrunning tricks), you can head straight to Hyrule Castle, either through the front door or from the back. There will be Guardians in your way and you might get blown up a lot, but it is possible to walk straight up to the castle from the Great Plateau without doing anything in between.
To start the battle with Calamity Ganon, you must make your way into the castle proper. You'll find an area named "Sanctum" and once you step in, you'll begin the battle with Calamity Ganon.
Just walk in and square up. Ride your horse on the southern side where the ruined town is, and ride straight through the big gates. Then follow the path. Dodge the lasers and the Apache-attack helicopter sentry shoota guys, then you're in. You never have to enter the castle till the Sanctum where the boss is (though you can to explore things).
This strat will have you fight 2 lynels on the way up as mini-bosses, but they're no issue for a pro who can fight Ganon-Guy.
The best strategy that I have found is to go through the docks in the back and go up through there and use a rune to move all the book cases for weapons then work your way up to the top of the library and go to the top of the castle and I have learned to get all the good weapons you can and fight every monster. Finally you are able to kill all the beasts in the castle that you will find the Gannon’s but if you have not beat the Gannon beasts you have to before you beat the calamity Gannon.