I freaking love it. The perpetual darkness surrounding it with the many many guardians on lookout give it a true final challenge atmosphere, while the music also has really hopeful vibes. In terms of overall atmosphere, it's my favourite final dungeon in the series now.
As far as gameplay is concerned, it does what the rest of the game does and gives you freedom to explore how you want to. There's no one entrance, there's no one path to ganon. There are many different underground tunnels and passageways in the Castle that allow you to approach it any way you want. This helps really make it feel like a castle, which most of the games fail at in that aspect. One thing I love in dungeons is believability, and wandering around Hyrule Castle, I really do get the sense that it was once a beautiful, grand seat of royalty.
But that's just the layout. The enemies are challenging, especially the lynels (which, if you are not expecting them, would certainly present a great challenge; I knew they were there though so I was prepared). To make it more difficult, I even got a blood moon after I had liberated most of the Castle, so all the enemies respawned, including the one lynel i had already fought, and several guardians that I had used up precious ancient arrows on.
So ya. Best dungeon in the game, best final dungeon in the series, and one of the best dungeons overall. Great atmosphere, challenging enemies, believable layout and design. Basically everything I look for in a dungeon. If there was one complaint I would have, it's that the final boss is a bit underwhelming (not Calamity Ganon, that fight is fantastic). Other than that, I really don't have anything to complain about. This is a truly great dungeon with really badass music.